#428

backend/app/controllers/application_controller.rb

@@ -1,6 +1,11 @@
 class ApplicationController < ActionController::API
+  class UnsupportedLocale < StandardError
+    ;
+  end
+
   rescue_from ActiveRecord::RecordInvalid, with: :render_record_invalid
   rescue_from ActiveRecord::RecordNotUnique, with: :render_record_not_unique
+  rescue_from(UnsupportedLocale) { head :not_found }
 
   before_action :reject_banned_ip_address!
   before_action :authenticate_user
@@ -93,5 +98,15 @@ class ApplicationController < ActionController::API
     value
   end
 
-  def resolve_locale! = Locale.find_by(code: params[:locale]) || Locale.nipponese
+  def resolve_locale!
+    code = params.fetch(:locale, 'ja')
+    unless code.is_a?(String) && Locale.codes.value?(code)
+      raise UnsupportedLocale
+    end
+
+    locale = Locale.find_by(code:)
+    raise UnsupportedLocale unless locale && locale.deprecated_at.nil?
+
+    locale
+  end
 end

backend/app/models/language.rb

@@ -1,5 +1,5 @@
 class Language < ApplicationRecord
   has_many :locales, foreign_key: :language_code, primary_key: :code
 
-  enum :code, ja: 'ja'
+  enum :code, ja: 'ja', ain: 'ain'
 end

backend/app/models/locale.rb

@@ -1,26 +1,20 @@
 class Locale < ApplicationRecord
-  after_create_commit :generate_tag_names!
+  after_save :generate_tag_names!, if: :needs_tag_names?
 
   belongs_to :language, foreign_key: :language_code, primary_key: :code
   belongs_to :script, foreign_key: :script_code, primary_key: :code
 
-  enum :code, ja: 'ja', en: 'en', fr: 'fr'
+  enum :code, ja: 'ja', ain_kana: 'ain-Kana'
 
   def self.nipponese = Locale.find('ja')
 
   private
 
+  def needs_tag_names?
+    deprecated_at.nil? && (saved_change_to_code? || saved_change_to_deprecated_at?)
+  end
+
   def generate_tag_names!
-    ApplicationRecord.transaction do
-      tag_ids = TagName.where(language_code:, primary_flg: true).pluck(:tag_id)
-      Tag.where.not(id: tag_ids).includes(:tag_names).find_each do
-        TagName.create!(tag_id: _1.id,
-                        language_code:,
-                        name: TagName.generate_name(self, _1, _1.tag_name('ja')),
-                        script_code:,
-                        primary_flg: true,
-                        auto_generated: true)
-      end
-    end
+    TagNameCompletion.for_locale!(self)
   end
 end

backend/app/models/script.rb

@@ -1,5 +1,5 @@
 class Script < ApplicationRecord
   has_many :locales, foreign_key: :script_code, primary_key: :code
 
-  enum :code, Jpan: 'Jpan'
+  enum :code, jpan: 'Jpan', kana: 'Kana'
 end

backend/app/models/tag.rb

@@ -111,7 +111,12 @@ class Tag < ApplicationRecord
   def has_deerjikists = deerjikists.loaded? ? deerjikists.any? : deerjikists.exists?
 
   def snapshot_aliases(language_code)
-    tag_names.where(language_code:, primary_flg: false).order(:name).pluck(:name)
+    if tag_names.loaded?
+      tag_names.select { _1.language_code == language_code && !(_1.primary?) }
+          .sort_by(&:name).map(&:name)
+    else
+      tag_names.where(language_code:, primary_flg: false).order(:name).pluck(:name)
+    end
   end
 
   def snapshot_parent_tag_ids = parents.order(:id).pluck(:id)
@@ -263,16 +268,19 @@ class Tag < ApplicationRecord
         next tn.tag if tn
 
         tag = Tag.create!(category:)
-        tn = TagName.create!(tag:, language_code:, name:,
-                             script_code: locale.script_code, primary_flg: true,
-                             auto_generated: false)
-        generate_tag_names!(locale, tag, tn)
+        TagName.create!(tag:, language_code:, name:,
+                        script_code: locale.script_code, primary_flg: true,
+                        auto_generated: false)
+        TagNameCompletion.for_tag!(tag)
         tag.association(:tag_names).reset
 
         tag
       end
     rescue ActiveRecord::RecordNotUnique
-      retry
+      existing = TagName.find_by(language_code:, name:)
+      raise unless existing
+
+      existing.tag
     end
 
     def merge_tags! target_tag, source_tags, created_by_user: nil
@@ -352,28 +360,6 @@ class Tag < ApplicationRecord
       total_s * 1_000 + match[:ms].to_s.ljust(3, '0')[0, 3].to_i
     end
 
-    private
-
-    def generate_tag_names! locale, tag, base_tag_name
-      ApplicationRecord.transaction do
-        locales =
-            Locale
-            .where(deprecated_at: nil)
-            .where.not(language_code: locale.language_code)
-            .order(:language_code, :created_at, :script_code)
-            .uniq { _1.language_code }
-        locales.each do |locale|
-          language_code = locale.language_code
-          script_code = locale.script_code
-          TagName.create!(tag_id: tag.id,
-                          language_code:,
-                          name: TagName.generate_name(locale, tag, base_tag_name),
-                          script_code:,
-                          primary_flg: true,
-                          auto_generated: true)
-        end
-      end
-    end
   end
 
   private

backend/app/models/tag_name.rb

@@ -34,9 +34,15 @@ class TagName < ApplicationRecord
   end
 
   def self.generate_name locale, tag, base_tag_name
-    # TODO: 言語ごとの自動命名ロジック完成したら書く．
+    fallback = "Tag_##{ tag.id }"
+    return fallback unless locale.language_code == 'ain' && base_tag_name&.language_code == 'ja'
 
-    "Tag_##{ tag.id }"
+    name = TagNameTranscription.ainu(base_tag_name.name)
+    candidate = new(tag:, name:, language_code: 'ain', script_code: 'Kana',
+                    primary_flg: true, auto_generated: true)
+    candidate.valid? ? name : fallback
+  rescue TagNameTranscription::Unconvertible
+    fallback
   end
 
   private
@@ -54,6 +60,8 @@ class TagName < ApplicationRecord
   end
 
   def name_must_be_sanitised
+    return if auto_generated && name == "Tag_##{ tag_id }"
+
     if name? && name != TagNameSanitisationRule.sanitise(name)
       errors.add :name, '名前に使用できない文字が含まれてゐます．'
     end

backend/app/services/tag_name_completion.rb

@@ -0,0 +1,43 @@
+class TagNameCompletion
+  def self.for_locale! locale
+    named_tags = TagName.where(language_code: locale.language_code, primary_flg: true)
+                       .select(:tag_id)
+    Tag.where.not(id: named_tags).find_each do |tag|
+      for_tag!(tag, locales: [locale])
+    end
+  end
+
+  def self.for_tag! tag, locales: nil
+    locales ||= Locale.where(deprecated_at: nil).order(:language_code, :created_at, :script_code)
+    tag.with_lock do
+      primary_names = tag.tag_names.where(primary_flg: true).index_by(&:language_code)
+      locales.to_a.uniq(&:language_code).each do |locale|
+        next if primary_names.key?(locale.language_code)
+
+        name = TagName.generate_name(locale, tag, primary_names['ja'])
+        primary_names[locale.language_code] = create_name!(locale, tag, name)
+      end
+      tag.association(:tag_names).reset
+    end
+  end
+
+  def self.create_name! locale, tag, name
+    persist_name!(locale, tag, name)
+  rescue ActiveRecord::RecordNotUnique, ActiveRecord::RecordInvalid => error
+    fallback = "Tag_##{ tag.id }"
+    collision = error.is_a?(ActiveRecord::RecordNotUnique) ||
+        error.record.errors.of_kind?(:name, :taken)
+    raise unless name != fallback && collision
+
+    persist_name!(locale, tag, fallback)
+  end
+  private_class_method :create_name!
+
+  def self.persist_name! locale, tag, name
+    TagName.transaction(requires_new: true) do
+      TagName.create!(tag:, name:, language_code: locale.language_code,
+                      script_code: locale.script_code, primary_flg: true, auto_generated: true)
+    end
+  end
+  private_class_method :persist_name!
+end

backend/app/services/tag_name_transcription.rb

@@ -0,0 +1,23 @@
+module TagNameTranscription
+  class Unconvertible < StandardError
+    ;
+  end
+
+  def self.ainu name
+    previous_script = nil
+    JapaneseSegments.split(name).map { |script, text|
+      boundary = previous_script && script != previous_script &&
+          JapaneseSegments.japanese?(previous_script) && JapaneseSegments.japanese?(script)
+      previous_script = script
+      reading = script == :kanji ? JisReadings.read(text) : text
+      output =
+          if script == :literal
+            text
+          else
+            Katakana.render(Phonology.adapt(reading))
+          end
+
+      boundary ? "_#{ output }" : output
+    }.join
+  end
+end

backend/app/services/tag_name_transcription/japanese_segments.rb

@@ -0,0 +1,30 @@
+module TagNameTranscription
+  module JapaneseSegments
+    JAPANESE_SCRIPTS = [:kanji, :hiragana, :katakana].freeze
+
+    def self.japanese?(script) = JAPANESE_SCRIPTS.include?(script)
+
+    def self.split name
+      segments = []
+      name.to_s.scan(/\X/).each do |character|
+        script = classify(character, segments.last&.first)
+        if segments.last&.first == script
+          segments.last[1] << character
+        else
+          segments << [script, character.dup]
+        end
+      end
+      segments
+    end
+
+    def self.classify character, previous_script
+      return previous_script if character == 'ー' && japanese?(previous_script)
+      return :kanji if character.match?(/\A\p{Han}\z/)
+      return :hiragana if character.match?(/\A\p{Hiragana}\p{M}*\z/)
+      return :katakana if character.match?(/\A\p{Katakana}\p{M}*\z/)
+      return :literal if character.match?(/\A(?:\p{Latin}\p{M}*|[!-~])\z/)
+
+      raise Unconvertible
+    end
+  end
+end

backend/app/services/tag_name_transcription/jis_readings.rb

@@ -0,0 +1,13 @@
+module TagNameTranscription
+  module JisReadings
+    SOURCE = Rails.root.join('config/tag_name_readings/jis_level_one.tsv')
+    READINGS = File.foreach(SOURCE, chomp: true).reject { _1.start_with?('#') }.to_h { |line|
+      character, reading, = line.split("\t")
+      [character, reading]
+    }.freeze
+
+    def self.read text
+      text.each_char.map { READINGS.fetch(_1) { raise Unconvertible } }.join
+    end
+  end
+end

backend/app/services/tag_name_transcription/katakana.rb

@@ -0,0 +1,24 @@
+module TagNameTranscription
+  module Katakana
+    ROWS = {
+        '' => 'アイウエオ',
+        'k' => 'カキクケコ',
+        's' => 'サシスセソ',
+        'c' => ['チャ', 'チ', 'チュ', 'チェ', 'チョ'],
+        't' => ['タ', 'ティ', 'トゥ', 'テ', 'ト'],
+        'n' => 'ナニヌネノ',
+        'h' => 'ハヒフヘホ',
+        'p' => 'パピプペポ',
+        'm' => 'マミムメモ',
+        'r' => 'ラリルレロ' }.freeze
+    SYLLABLES = ROWS.flat_map { |onset, letters|
+      kana = letters.is_a?(String) ? letters.chars : letters
+      ['a', 'i', 'u', 'e', 'o'].zip(kana).map { |vowel, letter| [onset + vowel, letter] }
+    }.to_h.merge(
+        'ya' => 'ヤ', 'yu' => 'ユ', 'yo' => 'ヨ', 'wa' => 'ワ',
+        'k' => 'ㇰ', 's' => 'ㇱ', 't' => 'ッ', 'n' => 'ン', 'm' => 'ㇺ', 'p' => 'ㇷ゚',
+        'r:a' => 'ㇻ', 'r:i' => 'ㇼ', 'r:u' => 'ㇽ', 'r:e' => 'ㇾ', 'r:o' => 'ㇿ').freeze
+
+    def self.render(sounds) = sounds.map { SYLLABLES.fetch(_1) }.join
+  end
+end

backend/app/services/tag_name_transcription/phonology.rb

@@ -0,0 +1,36 @@
+module TagNameTranscription
+  module Phonology
+    ROWS = {
+        'アイウエオ' => ['a', 'i', 'u', 'e', 'o'],
+        'カキクケコ' => ['ka', 'ki', 'ku', 'ke', 'ko'],
+        'ガギグゲゴ' => ['ka', 'ki', 'ku', 'ke', 'ko'],
+        'サシスセソ' => ['sa', 'si', 'su', 'se', 'so'],
+        'ザジズゼゾ' => ['ca', 'ci', 'cu', 'ce', 'co'],
+        'タチツテト' => ['ta', 'ci', 'cu', 'te', 'to'],
+        'ダヂヅデド' => ['ta', 'ci', 'cu', 'te', 'to'],
+        'ナニヌネノ' => ['na', 'ni', 'nu', 'ne', 'no'],
+        'ハヒフヘホ' => ['ha', 'hi', 'hu', 'he', 'ho'],
+        'バビブベボ' => ['pa', 'pi', 'pu', 'pe', 'po'],
+        'パピプペポ' => ['pa', 'pi', 'pu', 'pe', 'po'],
+        'マミムメモ' => ['ma', 'mi', 'mu', 'me', 'mo'],
+        'ヤユヨ' => ['ya', 'yu', 'yo'],
+        'ラリルレロ' => ['ra', 'ri', 'ru', 're', 'ro'],
+        'ワ' => ['wa'],
+        'ン' => ['n'],
+        'ッ' => ['t'],
+        'ㇰ' => ['k'],
+        'ㇱㇲ' => ['s', 's'],
+        'ㇳ' => ['t'],
+        'ㇴ' => ['n'],
+        'ㇺ' => ['m'],
+        'ㇻㇼㇽㇾㇿ' => ['r:a', 'r:i', 'r:u', 'r:e', 'r:o'] }.freeze
+    SOUNDS = ROWS.flat_map { |characters, sounds| characters.chars.zip(sounds) }.to_h.freeze
+
+    def self.adapt reading
+      kana = reading.unicode_normalize(:nfc).tr('ぁ-ゖ', 'ァ-ヶ').tr('ャュョ', 'ヤユヨ')
+      kana.delete('ー').scan(/\X/).map { |character|
+        character == 'ㇷ゚' ? 'p' : SOUNDS.fetch(character) { raise Unconvertible }
+      }
+    end
+  end
+end

backend/db/seeds.rb

@@ -8,6 +8,16 @@
 #     MovieGenre.find_or_create_by!(name: genre_name)
 #   end
 
+ApplicationRecord.transaction do
+  Language.find_or_create_by!(code: 'ain') { _1.name = 'ain' }
+  Script.find_or_create_by!(code: 'Kana') { _1.name = 'Kana' }
+  locale = Locale.find_or_initialize_by(code: 'ain-Kana')
+  locale.assign_attributes(language_code: 'ain', script_code: 'Kana', region_code: 392,
+                           variants: '', name: 'ain-Kana', deprecated_at: nil)
+  locale.save!
+  TagNameCompletion.for_locale!(locale)
+end
+
 post_url_sanitisation_rules = [
   { priority: 10,
     source_pattern: '\Ahttps?://youtu\.be/([^/?#]+)(?:[?#].*)?\z',

backend/spec/factories/tags.rb

@@ -24,17 +24,7 @@ FactoryBot.define do
 
       evaluator.primary_tag_name.update!(tag:, primary_flg: true)
 
-      Locale.where(deprecated_at: nil).order(:language_code, :created_at, :script_code).to_a
-          .uniq(&:language_code).each do |locale|
-        next if tag.tag_names.exists?(language_code: locale.language_code,
-                                      primary_flg: true)
-
-        create(:tag_name,
-            tag:, language_code: locale.language_code,
-            script_code: locale.script_code,
-            name: TagName.generate_name(locale, tag, evaluator.primary_tag_name),
-            primary_flg: true, auto_generated: true)
-      end
+      TagNameCompletion.for_tag!(tag)
     end
   end
 end

backend/spec/models/language_spec.rb

@@ -6,9 +6,8 @@ RSpec.describe Language, type: :model do
     expect(described_class.where(deprecated_at: nil).pluck(:code)).to eq(['ja'])
   end
 
-  it 'accepts English when multilingual support is enabled' do
-    pending '日本語以外の Language 有効化は別課題で実装する'
-    expect { described_class.create!(code: 'en', name: 'English') }
+  it 'accepts Ainu when multilingual support is enabled' do
+    expect { described_class.create!(code: 'ain', name: 'ain') }
         .to change(described_class, :count).by(1)
   end
 

backend/spec/models/locale_activation_spec.rb

@@ -0,0 +1,82 @@
+require 'rails_helper'
+
+RSpec.describe 'Locale activation', type: :model do
+  let!(:language) { Language.create!(code: 'ain', name: 'ain') }
+  let!(:script) { Script.create!(code: 'Kana', name: 'Kana') }
+  let!(:first) { create(:tag, primary_name: 'activation_first') }
+  let!(:second) { create(:tag, primary_name: 'activation_second') }
+
+  def build_locale deprecated_at: nil
+    Locale.new(code: 'ain-Kana', language_code: language.code,
+               script_code: script.code, region_code: 392, variants: '',
+               name: 'ain-Kana', deprecated_at:)
+  end
+
+  def generated_names
+    TagName.where(language_code: 'ain', primary_flg: true)
+  end
+
+  def fail_second_generation!
+    allow(TagName).to receive(:generate_name).and_call_original
+    allow(TagName).to receive(:generate_name)
+        .with(kind_of(Locale), second, second.tag_name('ja'))
+        .and_raise('backfill failed')
+  end
+
+  it 'rolls back a newly created locale and the preceding generated names' do
+    locale = build_locale
+    fail_second_generation!
+
+    expect { locale.save! }.to raise_error('backfill failed')
+
+    expect(Locale.exists?(code: 'ain-Kana')).to be(false)
+    expect(generated_names).to be_empty
+    expect(first.reload.name('ja')).to eq('activation_first')
+    expect(second.reload.name('ja')).to eq('activation_second')
+  end
+
+  it 'does not backfill while the locale is inactive' do
+    build_locale(deprecated_at: Time.current).save!
+
+    expect(generated_names).to be_empty
+  end
+
+  it 'backfills missing primary names when the locale becomes active' do
+    locale = build_locale(deprecated_at: Time.current)
+    locale.save!
+
+    locale.update!(deprecated_at: nil)
+
+    expect(generated_names.pluck(:tag_id)).to contain_exactly(first.id, second.id)
+    expect(generated_names).to all(have_attributes(
+        script_code: 'Kana', primary_flg: true, auto_generated: true))
+  end
+
+  it 'rolls back activation and all generated names when backfill fails' do
+    locale = build_locale(deprecated_at: Time.current)
+    locale.save!
+    previous_deprecated_at = locale.reload.deprecated_at
+    fail_second_generation!
+
+    expect { locale.update!(deprecated_at: nil) }.to raise_error('backfill failed')
+
+    expect(locale.reload.deprecated_at).to eq(previous_deprecated_at)
+    expect(generated_names).to be_empty
+  end
+
+  it 'preserves a human primary through deactivation and repeated activation' do
+    locale = build_locale
+    locale.save!
+    primary = generated_names.find_by!(tag: first)
+    primary.update!(name: 'human_confirmed', auto_generated: false)
+    snapshot = generated_names.order(:id).map(&:attributes)
+
+    2.times do
+      locale.update!(deprecated_at: Time.current)
+      locale.update!(deprecated_at: nil)
+    end
+
+    expect(generated_names.order(:id).map(&:attributes)).to eq(snapshot)
+    expect(primary.reload).to have_attributes(name: 'human_confirmed', auto_generated: false)
+  end
+end

backend/spec/models/locale_spec.rb

@@ -1,13 +1,13 @@
 require 'rails_helper'
 
 RSpec.describe Locale, type: :model do
-  include_context 'English locale'
+  include_context 'Ainu locale'
 
-  def create_english_locale!
-    prepare_english_reference!
-    described_class.create!(code: 'en', language_code: 'en',
-                            script_code: 'Latn', region_code: 826,
-                            variants: '', name: 'English')
+  def create_ainu_locale!
+    prepare_ainu_reference!
+    described_class.create!(code: 'ain-Kana', language_code: 'ain',
+                            script_code: 'Kana', region_code: 392,
+                            variants: '', name: 'Ainu')
   end
 
   it 'keeps the active Nipponese locale consistent with its identity tuple' do
@@ -27,7 +27,6 @@ RSpec.describe Locale, type: :model do
   end
 
   it 'generates a primary name for every existing tag using the name generator' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
     first = create(:tag, primary_name: 'first_existing_tag')
     second = create(:tag, primary_name: 'second_existing_tag')
     allow(TagName).to receive(:generate_name).and_call_original
@@ -35,63 +34,60 @@ RSpec.describe Locale, type: :model do
         .with(kind_of(described_class), first, primary_tag_name_for(first, 'ja'))
         .and_return('name_from_generator')
 
-    locale = create_english_locale!
+    locale = create_ainu_locale!
 
     expect(TagName).to have_received(:generate_name)
         .with(locale, first, primary_tag_name_for(first, 'ja'))
     expect(TagName).to have_received(:generate_name)
         .with(locale, second, primary_tag_name_for(second, 'ja'))
-    expect(first.tag_names.find_by!(language_code: 'en', primary_flg: true))
+    expect(first.tag_names.find_by!(language_code: 'ain', primary_flg: true))
         .to have_attributes(name: 'name_from_generator', tag_id: first.id,
-                            language_code: 'en', script_code: 'Latn',
+                            language_code: 'ain', script_code: 'Kana',
                             primary_flg: true, auto_generated: true)
-    expect(second.tag_names.find_by!(language_code: 'en', primary_flg: true))
-        .to have_attributes(tag_id: second.id, language_code: 'en',
-                            script_code: 'Latn', primary_flg: true,
+    expect(second.tag_names.find_by!(language_code: 'ain', primary_flg: true))
+        .to have_attributes(tag_id: second.id, language_code: 'ain',
+                            script_code: 'Kana', primary_flg: true,
                             auto_generated: true)
-    expect(TagName.where(language_code: 'en', primary_flg: true,
+    expect(TagName.where(language_code: 'ain', primary_flg: true,
                          tag_id: [first.id, second.id]).count).to eq(2)
   end
 
   it 'does not duplicate an existing primary name in the new language' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_reference!
+    prepare_ainu_reference!
     tag = create(:tag, primary_name: 'already_named')
-    existing = create(:tag_name, name: 'existing_english', tag:,
-                                 language_code: 'en', script_code: 'Latn')
+    existing = create(:tag_name, name: 'existing_ainu', tag:,
+                                 language_code: 'ain', script_code: 'Kana')
 
-    create_english_locale!
+    create_ainu_locale!
 
-    expect(TagName.where(tag_id: tag.id, language_code: 'en', primary_flg: true))
+    expect(TagName.where(tag_id: tag.id, language_code: 'ain', primary_flg: true))
         .to contain_exactly(existing)
     expect(existing.reload).to have_attributes(
-        tag_id: tag.id, language_code: 'en', script_code: 'Latn',
+        tag_id: tag.id, language_code: 'ain', script_code: 'Kana',
         primary_flg: true, auto_generated: false)
   end
 
   it 'creates a primary name when the language has only an alias' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_reference!
+    prepare_ainu_reference!
     tag = create(:tag, primary_name: 'alias_only_tag')
     alias_name = create(:tag_name, :alias, name: 'alias_en',
                                           tag:,
-                                          language_code: 'en', script_code: 'Latn')
+                                          language_code: 'ain', script_code: 'Kana')
 
-    create_english_locale!
+    create_ainu_locale!
 
     expect(alias_name.reload).to have_attributes(
         tag_id: tag.id,
-        language_code: 'en', script_code: 'Latn', primary_flg: false)
-    generated = TagName.find_by!(tag_id: tag.id, language_code: 'en', primary_flg: true)
-    expect(generated).to have_attributes(tag_id: tag.id, language_code: 'en',
-                                         script_code: 'Latn', primary_flg: true,
+        language_code: 'ain', script_code: 'Kana', primary_flg: false)
+    generated = TagName.find_by!(tag_id: tag.id, language_code: 'ain', primary_flg: true)
+    expect(generated).to have_attributes(tag_id: tag.id, language_code: 'ain',
+                                         script_code: 'Kana', primary_flg: true,
                                          auto_generated: true)
-    expect(TagName.where(tag_id: tag.id, language_code: 'en', primary_flg: true).count).to eq(1)
+    expect(TagName.where(tag_id: tag.id, language_code: 'ain', primary_flg: true).count).to eq(1)
   end
 
   it 'rolls back the locale and all generated names when backfill fails' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_reference!
+    prepare_ainu_reference!
     first = create(:tag, primary_name: 'atomic_first')
     second = create(:tag, primary_name: 'atomic_second')
     allow(TagName).to receive(:generate_name).and_call_original
@@ -99,31 +95,27 @@ RSpec.describe Locale, type: :model do
         .with(kind_of(described_class), second, primary_tag_name_for(second, 'ja'))
         .and_raise('backfill failed')
 
-    expect { create_english_locale! }.to raise_error('backfill failed')
+    expect { create_ainu_locale! }.to raise_error('backfill failed')
 
-    expect(described_class.exists?(code: 'en')).to be(false)
-    expect(TagName.where(language_code: 'en', tag_id: [first.id, second.id])).to be_empty
+    expect(described_class.exists?(code: 'ain-Kana')).to be(false)
+    expect(TagName.where(language_code: 'ain', tag_id: [first.id, second.id])).to be_empty
   end
 
-  it 'accepts a stable locale code outside the historical enum values' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_reference!
+  it 'rejects a private locale code outside the supported locale set' do
+    prepare_ainu_reference!
 
-    locale = described_class.create!(
-        code: 'en-x-rspec', language_code: 'en',
-        script_code: 'Latn', region_code: 826,
-        variants: 'x-rspec', name: 'English')
-
-    expect(locale.reload).to have_attributes(
-        code: 'en-x-rspec', language_code: 'en', script_code: 'Latn',
-        region_code: 826, variants: 'x-rspec')
+    expect {
+      described_class.create!(
+          code: 'ain-Kana-x-rspec', language_code: 'ain',
+          script_code: 'Kana', region_code: 392,
+          variants: 'x-rspec', name: 'Ainu')
+    }.to raise_error(ArgumentError)
   end
 
-  it 'registers the future British English locale tuple' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
+  it 'registers the supported Ainu locale tuple' do
 
-    expect(locale).to have_attributes(code: 'en', language_code: 'en',
-                                    script_code: 'Latn', region_code: 826,
+    expect(locale).to have_attributes(code: 'ain-Kana', language_code: 'ain',
+                                    script_code: 'Kana', region_code: 392,
                                     variants: '')
   end
 end

backend/spec/models/script_spec.rb

@@ -1,7 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe Script, type: :model do
-  include_context 'English locale'
+  include_context 'Ainu locale'
 
   it 'selects locales by script code, not language code' do
     expect(described_class.find('Jpan').locales).to contain_exactly(Locale.nipponese)
@@ -25,11 +25,10 @@ RSpec.describe Script, type: :model do
     }
   end
 
-  it 'selects Latin locales when multilingual support is enabled' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
+  it 'selects Kana locales when multilingual support is enabled' do
     locale
 
-    expect(described_class.find('Latn').locales).to contain_exactly(locale)
+    expect(described_class.find('Kana').locales).to contain_exactly(locale)
     expect(described_class.find('Jpan').locales).not_to include(locale)
   end
 end

backend/spec/models/tag_name_sanitisation_rule_spec.rb

@@ -95,38 +95,36 @@ RSpec.describe TagNameSanitisationRule, type: :model do
     end
 
     context 'when only another language has the sanitised name' do
-      include_context 'English locale'
+      include_context 'Ainu locale'
 
       it 'keeps both names when sanitisation produces a cross-language match' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         nipponese = create(:tag_name, name: 'foobar')
-        english = create(:tag_name, name: 'temporary',
-                                   language_code: 'en', script_code: 'Latn')
-        english.update_columns(name: 'foo_bar')
+        ainu = create(:tag_name, name: 'temporary',
+                                   language_code: 'ain', script_code: 'Kana')
+        ainu.update_columns(name: 'foo_bar')
 
         expect { described_class.apply! }.not_to change(TagName, :count)
 
-        expect(english.reload).to have_attributes(name: 'foobar', language_code: 'en')
+        expect(ainu.reload).to have_attributes(name: 'foobar', language_code: 'ain')
         expect(nipponese.reload).to have_attributes(name: 'foobar', language_code: 'ja')
       end
 
       it 'renames without merging tags or deleting either language identity' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         nipponese = create(:tag, primary_name: 'foobar')
-        english_name = create(:tag_name, name: 'temporary',
-                                        language_code: 'en', script_code: 'Latn')
-        english = create(:tag, primary_tag_name: english_name)
-        english_name.update_columns(name: 'foo_bar')
+        ainu_name = create(:tag_name, name: 'temporary',
+                                        language_code: 'ain', script_code: 'Kana')
+        ainu = create(:tag, primary_tag_name: ainu_name)
+        ainu_name.update_columns(name: 'foo_bar')
 
         described_class.apply!
 
-        expect(english_name.reload).to have_attributes(
-            name: 'foobar', language_code: 'en', tag_id: english.id)
+        expect(ainu_name.reload).to have_attributes(
+            name: 'foobar', language_code: 'ain', tag_id: ainu.id)
         expect(primary_tag_name_for(nipponese.reload, 'ja')).to have_attributes(
             name: 'foobar', language_code: 'ja', tag_id: nipponese.id)
-        expect(primary_tag_name_for(english.reload, 'en')).to eq(english_name)
+        expect(primary_tag_name_for(ainu.reload, 'ain')).to eq(ainu_name)
       end
     end
 

backend/spec/models/tag_name_spec.rb

@@ -1,17 +1,16 @@
 require 'rails_helper'
 
 RSpec.describe TagName, type: :model do
-  include_context 'English locale'
+  include_context 'Ainu locale'
 
   it 'allows the same name in different languages' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
     locale
     create(:tag_name, name: 'shared_name')
-    english = build(:tag_name, name: 'shared_name',
-                              language_code: 'en', script_code: 'Latn')
+    ainu = build(:tag_name, name: 'shared_name',
+                              language_code: 'ain', script_code: 'Kana')
 
-    expect(english).to be_valid
-    expect { english.save! }.to change(described_class, :count).by(1)
+    expect(ainu).to be_valid
+    expect { ainu.save! }.to change(described_class, :count).by(1)
   end
 
   it 'rejects a duplicate name within the same language' do
@@ -67,11 +66,10 @@ RSpec.describe TagName, type: :model do
   end
 
   it 'completes an ordinary tag factory under future active languages' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
     locale
     tag = create(:tag, primary_name: 'future_factory')
 
-    expect(TagName.where(tag:, language_code: 'en', primary_flg: true))
+    expect(TagName.where(tag:, language_code: 'ain', primary_flg: true))
         .to contain_exactly(have_attributes(auto_generated: true))
   end
 
@@ -114,46 +112,44 @@ RSpec.describe TagName, type: :model do
     end
 
     it 'resolves the primary name of the alias language on the same tag' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       tag = create(:tag, primary_name: '日本語正本')
-      english_primary = primary_tag_name_for(tag, 'en')
-      english_primary.update!(name: 'english_primary', auto_generated: false)
-      english_alias = create(:tag_name, :alias, tag:, name: 'english_alias',
-                                                 language_code: 'en', script_code: 'Latn')
+      ainu_primary = primary_tag_name_for(tag, 'ain')
+      ainu_primary.update!(name: 'ainu_primary', auto_generated: false)
+      ainu_alias = create(:tag_name, :alias, tag:, name: 'ainu_alias',
+                                                 language_code: 'ain', script_code: 'Kana')
       representative_id = tag.tag_name('ja')&.id
 
-      primary = described_class.find_by!(tag_id: english_alias.tag_id,
-                                         language_code: english_alias.language_code,
+      primary = described_class.find_by!(tag_id: ainu_alias.tag_id,
+                                         language_code: ainu_alias.language_code,
                                          primary_flg: true)
 
-      expect(primary).to eq(english_primary)
-      expect(described_class.canonicalise(locale, [english_alias.name]))
-          .to eq([english_primary.name])
+      expect(primary).to eq(ainu_primary)
+      expect(described_class.canonicalise(locale, [ainu_alias.name]))
+          .to eq([ainu_primary.name])
       expect(tag.reload.tag_name('ja')&.id).to eq(representative_id)
     end
 
     it 'resolves only aliases in the requested language' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       nipponese = create(:tag, primary_name: 'nipponese_canonical')
-      english_name = create(:tag_name, name: 'english_canonical',
-                                      language_code: 'en', script_code: 'Latn')
-      english = create(:tag, primary_tag_name: english_name)
+      ainu_name = create(:tag_name, name: 'ainu_canonical',
+                                      language_code: 'ain', script_code: 'Kana')
+      ainu = create(:tag, primary_tag_name: ainu_name)
       nipponese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                   tag: nipponese)
-      english_alias = create(:tag_name, :alias, name: 'shared_alias',
-                                                tag: english,
-                                                language_code: 'en', script_code: 'Latn')
+      ainu_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                                tag: ainu,
+                                                language_code: 'ain', script_code: 'Kana')
 
       expect(nipponese_alias.reload).to have_attributes(
           tag_id: nipponese.id,
           primary_flg: false, language_code: 'ja')
-      expect(english_alias.reload).to have_attributes(
-          tag_id: english.id,
-          primary_flg: false, language_code: 'en')
+      expect(ainu_alias.reload).to have_attributes(
+          tag_id: ainu.id,
+          primary_flg: false, language_code: 'ain')
       expect(described_class.canonicalise(locale, ['shared_alias']))
-          .to eq(['english_canonical'])
+          .to eq(['ainu_canonical'])
       expect(described_class.canonicalise(Locale.nipponese, ['shared_alias']))
           .to eq(['nipponese_canonical'])
     end

backend/spec/models/tag_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe Tag, type: :model do
     it 'checks the requested language and treats an empty description as present' do
       TagDescription.create!(tag:, language_code: 'ja', body: '', version_no: 1)
       expect(tag.has_description?(language_code: 'ja')).to be(true)
-      expect(tag.has_description?(language_code: 'en')).to be(false)
+      expect(tag.has_description?(language_code: 'ain')).to be(false)
     end
 
     it 'returns false without a description' do
@@ -23,16 +23,16 @@ RSpec.describe Tag, type: :model do
       subscriber = ->(*args) { queries << args.last.fetch(:sql) }
       ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') do
         expect(loaded.has_description?(language_code: 'ja')).to be(true)
-        expect(loaded.has_description?(language_code: 'en')).to be(false)
+        expect(loaded.has_description?(language_code: 'ain')).to be(false)
       end
       expect(queries).to be_empty
     end
 
     it 'keeps language selection generic without enabling another Language record' do
       tag.association(:tag_descriptions).target = [
-          TagDescription.new(tag:, language_code: 'en', body: '', version_no: 1)]
+          TagDescription.new(tag:, language_code: 'ain', body: '', version_no: 1)]
       tag.association(:tag_descriptions).loaded!
-      expect(tag.has_description?(language_code: 'en')).to be(true)
+      expect(tag.has_description?(language_code: 'ain')).to be(true)
       expect(tag.has_description?(language_code: 'ja')).to be(false)
     end
   end
@@ -61,33 +61,32 @@ RSpec.describe Tag, type: :model do
   end
 
   describe '.normalise_tags!' do
-    include_context 'English locale'
+    include_context 'Ainu locale'
 
     it 'canonicalises aliases in the supplied language only' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
-      prepare_english_reference!
+      prepare_ainu_reference!
       nipponese = create(:tag, primary_name: 'nipponese_canonical')
-      english_name = create(:tag_name, name: 'english_canonical',
-                                      language_code: 'en', script_code: 'Latn')
-      english = create(:tag, primary_tag_name: english_name)
-      primary_tag_name_for(english, 'ja').update!(
-          name: 'english_in_nipponese', auto_generated: false)
-      english_locale = locale
+      ainu_name = create(:tag_name, name: 'ainu_canonical',
+                                      language_code: 'ain', script_code: 'Kana')
+      ainu = create(:tag, primary_tag_name: ainu_name)
+      primary_tag_name_for(ainu, 'ja').update!(
+          name: 'ainu_in_nipponese', auto_generated: false)
+      ainu_locale = locale
       nipponese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                   tag: nipponese)
-      english_alias = create(:tag_name, :alias, name: 'shared_alias',
-                                                tag: english,
-                                                language_code: 'en', script_code: 'Latn')
+      ainu_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                                tag: ainu,
+                                                language_code: 'ain', script_code: 'Kana')
 
       expect(nipponese_alias.reload).to have_attributes(
           tag_id: nipponese.id,
           primary_flg: false, language_code: 'ja')
-      expect(english_alias.reload).to have_attributes(
-          tag_id: english.id,
-          primary_flg: false, language_code: 'en')
+      expect(ainu_alias.reload).to have_attributes(
+          tag_id: ainu.id,
+          primary_flg: false, language_code: 'ain')
       expect(described_class.normalise_tags!(
-          english_locale, ['shared_alias'],
-          with_tagme: false, with_no_deerjikist: false)).to eq([english])
+          ainu_locale, ['shared_alias'],
+          with_tagme: false, with_no_deerjikist: false)).to eq([ainu])
       expect(described_class.normalise_tags!(
           Locale.nipponese, ['shared_alias'],
           with_tagme: false, with_no_deerjikist: false)).to eq([nipponese])
@@ -253,129 +252,120 @@ RSpec.describe Tag, type: :model do
 
   describe '.find_or_create_by_tag_name!' do
     context 'with an explicit locale' do
-      include_context 'English locale'
+      include_context 'Ainu locale'
 
       it 'creates the representative name with locale attributes and ownership' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         tag = described_class.find_or_create_by_tag_name!(
-            locale, 'english_name', category: :character)
+            locale, 'ainu_name', category: :character)
 
-        expect(primary_tag_name_for(tag, 'en')).to have_attributes(
-            language_code: 'en', script_code: 'Latn',
+        expect(primary_tag_name_for(tag, 'ain')).to have_attributes(
+            language_code: 'ain', script_code: 'Kana',
             primary_flg: true, tag_id: tag.id)
         expect(tag.category).to eq('character')
       end
 
-      it 'uses the English primary as the source for a generated Nipponese name' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
+      it 'uses a Japanese fallback for an Ainu-origin tag without reverse transcription' do
         locale
-        allow(TagName).to receive(:generate_name).and_return('generated_nipponese')
+        allow(TagName).to receive(:generate_name).and_call_original
 
         tag = described_class.find_or_create_by_tag_name!(
-            locale, 'english_origin', category: :general)
+            locale, 'ainu_origin', category: :general)
 
-        expect(TagName.where(tag:, language_code: 'en', primary_flg: true))
+        expect(TagName.where(tag:, language_code: 'ain', primary_flg: true))
             .to contain_exactly(have_attributes(
-                name: 'english_origin', auto_generated: false))
+                name: 'ainu_origin', auto_generated: false))
         expect(TagName.where(tag:, language_code: 'ja', primary_flg: true))
             .to contain_exactly(have_attributes(
-                name: 'generated_nipponese', auto_generated: true))
-        source_tag_name = TagName.find_by!(tag:, language_code: 'en', primary_flg: true)
+                name: "Tag_##{ tag.id }", auto_generated: true))
         expect(TagName).to have_received(:generate_name)
-            .with(Locale.nipponese, tag, source_tag_name)
+            .with(Locale.nipponese, tag, nil)
       end
 
       it 'gives a new tag one primary name for every active language' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         tag = described_class.find_or_create_by_tag_name!(
             Locale.nipponese, 'new_multilingual_tag', category: :general)
 
         expect(TagName.where(tag:, language_code: 'ja', primary_flg: true).count).to eq(1)
-        expect(TagName.where(tag:, language_code: 'en', primary_flg: true).count).to eq(1)
+        expect(TagName.where(tag:, language_code: 'ain', primary_flg: true).count).to eq(1)
       end
 
-      it 'creates one primary per language when multiple locales use English' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
+      it 'creates one primary per language when multiple locales use Ainu' do
         locale
         described_class.find_or_create_by_tag_name!(
-            locale, 'before_second_english_locale', category: :general)
-        Locale.create!(code: 'en-x-rspec', language_code: 'en',
-                       script_code: 'Latn', region_code: 826,
-                       variants: 'x-rspec', name: 'English')
+            locale, 'before_second_ainu_locale', category: :general)
 
         tag = described_class.find_or_create_by_tag_name!(
-            locale, 'one_english_primary', category: :general)
+            locale, 'one_ainu_primary', category: :general)
+        TagNameCompletion.for_tag!(tag, locales: [locale, locale.dup, Locale.nipponese])
 
-        expect(TagName.where(tag:, language_code: 'en', primary_flg: true).count).to eq(1)
+        expect(TagName.where(tag:, language_code: 'ain', primary_flg: true).count).to eq(1)
         expect(TagName.where(tag:, language_code: 'ja', primary_flg: true).count).to eq(1)
       end
 
       it 'rolls back a new tag and requested name if name generation fails' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         original_tag_count = described_class.count
         allow(TagName).to receive(:generate_name).and_raise('generation failed')
 
         expect {
           described_class.find_or_create_by_tag_name!(
-              locale, 'atomic_english_name', category: :general)
+              locale, 'atomic_ainu_name', category: :general)
         }.to raise_error('generation failed')
 
         expect(described_class.count).to eq(original_tag_count)
-        expect(TagName.where(language_code: 'en', name: 'atomic_english_name')).to be_empty
+        expect(TagName.where(language_code: 'ain', name: 'atomic_ainu_name')).to be_empty
         expect(described_class.joins(:tag_names)
-            .where(tag_names: { name: 'atomic_english_name' })).to be_empty
+            .where(tag_names: { name: 'atomic_ainu_name' })).to be_empty
       end
 
 
 
       it 'creates separate tag identities for the same name in different languages' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         nipponese = described_class.find_or_create_by_tag_name!(
             Locale.nipponese, 'same_name', category: :general)
-        english = described_class.find_or_create_by_tag_name!(
+        primary_tag_name_for(nipponese, 'ain').update!(
+            name: 'other_ainu_name', auto_generated: false)
+        ainu = described_class.find_or_create_by_tag_name!(
             locale, 'same_name', category: :character)
 
-        expect(english).not_to eq(nipponese)
-        expect(primary_tag_name_for(english, 'en')).to have_attributes(
-            name: 'same_name', tag_id: english.id, language_code: 'en', primary_flg: true)
+        expect(ainu).not_to eq(nipponese)
+        expect(primary_tag_name_for(ainu, 'ain')).to have_attributes(
+            name: 'same_name', tag_id: ainu.id, language_code: 'ain', primary_flg: true)
         expect(primary_tag_name_for(nipponese, 'ja')).to have_attributes(
             name: 'same_name', tag_id: nipponese.id, language_code: 'ja', primary_flg: true)
         expect(described_class.find_or_create_by_tag_name!(
-            locale, 'same_name', category: :general)).to eq(english)
+            locale, 'same_name', category: :general)).to eq(ainu)
         expect(described_class.find_or_create_by_tag_name!(
             Locale.nipponese, 'same_name', category: :general)).to eq(nipponese)
       end
 
       it 'preserves the V1 representative when looking up another primary language' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         tag = create(:tag, primary_name: '日本語代表名')
         representative_id = tag.tag_name('ja')&.id
-        english = primary_tag_name_for(tag, 'en')
-        english.update!(name: 'english_primary', auto_generated: false)
+        ainu = primary_tag_name_for(tag, 'ain')
+        ainu.update!(name: 'ainu_primary', auto_generated: false)
 
         found = described_class.find_or_create_by_tag_name!(
-            locale, english.name, category: :general)
+            locale, ainu.name, category: :general)
 
         expect(found).to eq(tag)
         expect(tag.reload.tag_name('ja')&.id).to eq(representative_id)
-        expect(english.reload.tag_id).to eq(tag.id)
+        expect(ainu.reload.tag_id).to eq(tag.id)
       end
 
       it 'normalises names using the supplied locale' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         tags = described_class.normalise_tags!(
-            locale, ['character:normalised_english'],
+            locale, ['character:normalised_ainu'],
             with_tagme: false, with_no_deerjikist: false)
 
         expect(tags.length).to eq(1)
-        expect(primary_tag_name_for(tags.first, 'en')).to have_attributes(
-            name: 'normalised_english', language_code: 'en', script_code: 'Latn',
+        expect(primary_tag_name_for(tags.first, 'ain')).to have_attributes(
+            name: 'normalised_ainu', language_code: 'ain', script_code: 'Kana',
             primary_flg: true, tag_id: tags.first.id)
       end
     end
@@ -444,26 +434,25 @@ RSpec.describe Tag, type: :model do
     end
 
     context 'with names in multiple languages' do
-      include_context 'English locale'
+      include_context 'Ainu locale'
 
       it 'keeps the target primary and moves source names as aliases' do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
-        english_primary = primary_tag_name_for(target_tag, 'en')
-        source_primary = primary_tag_name_for(source_tag, 'en')
+        ainu_primary = primary_tag_name_for(target_tag, 'ain')
+        source_primary = primary_tag_name_for(source_tag, 'ain')
         source_alias = create(:tag_name, :alias, tag: source_tag,
-            name: 'source_english_alias', language_code: 'en', script_code: 'Latn')
+            name: 'source_ainu_alias', language_code: 'ain', script_code: 'Kana')
 
-        expect([english_primary, source_primary]).to all(have_attributes(
+        expect([ainu_primary, source_primary]).to all(have_attributes(
             primary_flg: true, auto_generated: true))
 
         described_class.merge_tags!(target_tag, [source_tag])
 
-        expect(primary_tag_name_for(target_tag.reload, 'en')).to eq(english_primary)
-        expect(TagName.where(tag: target_tag, language_code: 'en', primary_flg: true))
-            .to contain_exactly(english_primary)
+        expect(primary_tag_name_for(target_tag.reload, 'ain')).to eq(ainu_primary)
+        expect(TagName.where(tag: target_tag, language_code: 'ain', primary_flg: true))
+            .to contain_exactly(ainu_primary)
         expect([source_primary.reload, source_alias.reload]).to all(have_attributes(
-            tag_id: target_tag.id, language_code: 'en', primary_flg: false))
+            tag_id: target_tag.id, language_code: 'ain', primary_flg: false))
         expect(TagName.where(tag: target_tag, language_code: 'ja', primary_flg: true))
             .to contain_exactly(primary_tag_name_for(target_tag, 'ja'))
         expect(source_tag_name.reload).to have_attributes(

backend/spec/requests/locale_resolution_spec.rb

@@ -0,0 +1,54 @@
+require 'rails_helper'
+
+RSpec.describe 'API locale resolution', type: :request do
+  let!(:tag) { create(:tag, primary_name: '日本語正本') }
+
+  it 'defaults an omitted locale to Japanese' do
+    get "/tags/#{ tag.id }"
+
+    expect(response).to have_http_status(:ok)
+    expect(json.fetch('name')).to eq('日本語正本')
+  end
+
+  it 'accepts an explicitly requested active Japanese locale' do
+    get "/tags/#{ tag.id }", params: { locale: 'ja' }
+
+    expect(response).to have_http_status(:ok)
+    expect(json.fetch('name')).to eq('日本語正本')
+  end
+
+  ['', 'en', 'ain', 'Jpan', 'JA', 'unknown', ['ja']].each do |code|
+    it "rejects invalid locale #{ code.inspect } instead of defaulting to Japanese" do
+      get "/tags/#{ tag.id }", params: { locale: code }
+
+      expect(response).to have_http_status(:not_found)
+      expect(response.body).not_to include('日本語正本')
+    end
+  end
+
+  it 'rejects a supported locale that has not been registered' do
+    get "/tags/#{ tag.id }", params: { locale: 'ain-Kana' }
+
+    expect(response).to have_http_status(:not_found)
+  end
+
+  it 'rejects an inactive locale' do
+    Locale.nipponese.update!(deprecated_at: Time.current)
+
+    get "/tags/#{ tag.id }", params: { locale: 'ja' }
+
+    expect(response).to have_http_status(:not_found)
+  end
+
+  it 'does not write posts or tag names under an invalid locale' do
+    sign_in_as(create_member_user!)
+
+    expect do
+      post '/posts', params: {
+          locale: 'en', title: 'invalid locale', url: 'https://example.com/invalid-locale',
+          tags: 'unwritten_tag' }
+    end.to change(Post, :count).by(0).and change(TagName, :count).by(0)
+
+    expect(response).to have_http_status(:not_found)
+  end
+end

backend/spec/requests/locale_write_paths_spec.rb

@@ -1,7 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe 'Locale propagation on write paths', type: :request do
-  include_context 'English locale'
+  include_context 'Ainu locale'
 
   let(:member) { create(:user, :member) }
 
@@ -54,13 +54,12 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
     end
   end
 
-  ['en', nil].each do |requested_locale|
+  ['ain-Kana', nil].each do |requested_locale|
     context "with locale #{ requested_locale.inspect }" do
       let(:locale_params) { requested_locale ? { locale: requested_locale } : { } }
       let(:expected_locale) { requested_locale ? locale : Locale.nipponese }
 
       it 'creates post tags in the requested language or Nipponese fallback' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         post '/posts', params: locale_params.merge(
             title: 'Locale post', url: 'https://example.com/locale-post',
@@ -72,7 +71,6 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
 
       it 'updates post tags in the requested language or Nipponese fallback' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         record = create(:post)
         PostVersionRecorder.record!(post: record, event_type: :create, created_by_user: member)
@@ -87,7 +85,6 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
 
       it 'passes the resolved locale to bulk creation' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         # Worker propagation is exercised in post_bulk_creator_spec.
         expect_any_instance_of(PostBulkCreator).to receive(:run)
@@ -102,7 +99,6 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
 
       it 'creates materials with tags in the resolved locale' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         post '/materials', params: locale_params.merge(
             tag: 'locale_material', url: 'https://example.com/material')
@@ -113,7 +109,6 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
 
       it 'updates materials with tags in the resolved locale' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         tag = create(:tag, category: :material)
         material = Material.create!(tag:, url: 'https://example.com/material')
@@ -127,7 +122,6 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
 
       it 'links Nico external tags to internal tags in the resolved locale' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         external = create(:external_tag)
         NicoTagVersionRecorder.record!(
@@ -141,7 +135,6 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
 
       it 'creates parent tags in the resolved locale' do
-        pending '日本語以外の Locale 有効化は別課題で実装する' if requested_locale
         locale if requested_locale
         tag = create(:tag)
 
@@ -157,10 +150,10 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
   end
 
   context 'with existing names in both languages' do
-    let(:english_name) do
-      create(:tag_name, name: 'shared_name', language_code: 'en', script_code: 'Latn')
+    let(:ainu_name) do
+      create(:tag_name, name: 'shared_name', language_code: 'ain', script_code: 'Kana')
     end
-    let(:english_tag) { create(:tag, primary_tag_name: english_name, category: :material) }
+    let(:ainu_tag) { create(:tag, primary_tag_name: ainu_name, category: :material) }
     let(:nipponese_tag) do
       create(:tag, primary_name: 'temporary_nipponese', category: :general).tap do |tag|
         # Isolate lookup from the separately tested uniqueness validation.
@@ -168,173 +161,163 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       end
     end
 
-    it 'plans and creates the post using the English identity and category' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
+    it 'plans and creates the post using the Ainu identity and category' do
       locale
-      english_tag
+      ainu_tag
       nipponese_tag
       post '/posts', params: {
-          locale: 'en', title: 'English identity',
-          url: 'https://example.com/english-identity', tags: 'shared_name', parent_post_ids: '' }
+          locale: 'ain-Kana', title: 'Ainu identity',
+          url: 'https://example.com/ainu-identity', tags: 'shared_name', parent_post_ids: '' }
 
       expect(response).to have_http_status(:created), response.body
       record = Post.find(json.fetch('id'))
-      expect(record.tags).to include(english_tag)
+      expect(record.tags).to include(ainu_tag)
       expect(record.tags).not_to include(nipponese_tag)
-      expect(english_tag.reload.category).to eq('material')
+      expect(ainu_tag.reload.category).to eq('material')
       expect(nipponese_tag.reload.category).to eq('general')
     end
 
-    it 'updates the post using the English identity' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
+    it 'updates the post using the Ainu identity' do
       locale
-      english_tag
+      ainu_tag
       nipponese_tag
       record = create(:post)
       PostVersionRecorder.record!(post: record, event_type: :create, created_by_user: member)
 
       put "/posts/#{ record.id }", params: {
-          locale: 'en', title: record.title, tags: 'shared_name', parent_post_ids: '',
+          locale: 'ain-Kana', title: record.title, tags: 'shared_name', parent_post_ids: '',
           base_version_no: record.reload.version_no }
 
       expect(response).to have_http_status(:ok), response.body
-      expect(record.reload.tags).to include(english_tag)
+      expect(record.reload.tags).to include(ainu_tag)
       expect(record.tags).not_to include(nipponese_tag)
     end
 
-    it 'resolves the English material tag' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
+    it 'resolves the Ainu material tag' do
       locale
-      english_tag
+      ainu_tag
       nipponese_tag
       post '/materials', params: {
-          locale: 'en', tag: 'shared_name', url: 'https://example.com/english-material' }
+          locale: 'ain-Kana', tag: 'shared_name', url: 'https://example.com/ainu-material' }
 
       expect(response).to have_http_status(:created), response.body
-      expect(Material.find(json.fetch('id')).tag).to eq(english_tag)
+      expect(Material.find(json.fetch('id')).tag).to eq(ainu_tag)
       expect(Material.where(tag: nipponese_tag)).to be_empty
     end
 
-    it 'resolves the English parent tag' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
+    it 'resolves the Ainu parent tag' do
       locale
-      english_tag
+      ainu_tag
       nipponese_tag
       child = create(:tag)
 
       put "/tags/#{ child.id }", params: {
-          locale: 'en', name: primary_tag_name_for(child, 'ja').name,
+          locale: 'ain-Kana', name: primary_tag_name_for(child, 'ja').name,
           category: child.category, deprecated: false,
           aliases: '', parent_tags: 'shared_name' }
 
       expect(response).to have_http_status(:ok), response.body
-      expect(TagImplication.where(tag: child).pluck(:parent_tag_id)).to eq([english_tag.id])
+      expect(TagImplication.where(tag: child).pluck(:parent_tag_id)).to eq([ainu_tag.id])
     end
 
-    it 'resolves the English internal tag for a Nico link' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
+    it 'resolves the Ainu internal tag for a Nico link' do
       locale
-      english_tag
+      ainu_tag
       nipponese_tag
       external = create(:external_tag)
       NicoTagVersionRecorder.record!(
           external_tag: external, event_type: :create, created_by_user: member)
 
-      put "/tags/nico/#{ external.id }", params: { locale: 'en', tags: 'shared_name' }
+      put "/tags/nico/#{ external.id }", params: { locale: 'ain-Kana', tags: 'shared_name' }
 
       expect(response).to have_http_status(:ok), response.body
-      expect(external.reload.linked_tags).to contain_exactly(english_tag)
+      expect(external.reload.linked_tags).to contain_exactly(ainu_tag)
     end
   end
 
-  context 'when renaming or updating aliases in English' do
-    let(:english_name) do
-      create(:tag_name, name: 'english_original', language_code: 'en', script_code: 'Latn')
+  context 'when renaming or updating aliases in Ainu' do
+    let(:ainu_name) do
+      create(:tag_name, name: 'ainu_original', language_code: 'ain', script_code: 'Kana')
     end
-    let(:tag) { create(:tag, primary_tag_name: english_name) }
+    let(:tag) { create(:tag, primary_tag_name: ainu_name) }
 
     it 'marks a human-renamed generated primary as no longer auto-generated' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       generated_tag = create(:tag, primary_name: 'rename_generated_source')
-      generated_name = primary_tag_name_for(generated_tag, 'en')
+      generated_name = primary_tag_name_for(generated_tag, 'ain')
       expect(generated_name.auto_generated).to be(true)
 
       patch "/tags/#{ generated_tag.id }", params: {
-          locale: 'en', name: 'human_english_name' }
+          locale: 'ain-Kana', name: 'human_ainu_name' }
 
       expect(response).to have_http_status(:ok), response.body
       expect(generated_name.reload).to have_attributes(
-          name: 'human_english_name', auto_generated: false)
+          name: 'human_ainu_name', auto_generated: false)
     end
 
     it 'does not steal another tag primary for an alias' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       other_tag = create(:tag, primary_name: 'other_nipponese')
-      other_primary = primary_tag_name_for(other_tag, 'en')
-      other_primary.update!(name: 'other_english_primary', auto_generated: false)
-      create(:tag_name, :alias, tag:, name: 'existing_english_alias',
-                                language_code: 'en', script_code: 'Latn')
-      primary_name_before = primary_tag_name_for(tag, 'en').name
+      other_primary = primary_tag_name_for(other_tag, 'ain')
+      other_primary.update!(name: 'other_ainu_primary', auto_generated: false)
+      create(:tag_name, :alias, tag:, name: 'existing_ainu_alias',
+                                language_code: 'ain', script_code: 'Kana')
+      primary_name_before = primary_tag_name_for(tag, 'ain').name
       aliases_before =
-          tag.tag_names.where(language_code: 'en', primary_flg: false)
+          tag.tag_names.where(language_code: 'ain', primary_flg: false)
               .order(:name).pluck(:name)
       category_before = tag.category
 
       put "/tags/#{ tag.id }", params: {
-          locale: 'en', name: 'renamed_before_alias_collision',
+          locale: 'ain-Kana', name: 'renamed_before_alias_collision',
           category: 'character', deprecated: false,
-          aliases: 'other_english_primary', parent_tags: '' }
+          aliases: 'other_ainu_primary', parent_tags: '' }
 
       aggregate_failures do
         expect(response).to have_http_status(:unprocessable_entity)
         expect(other_primary.reload).to have_attributes(
-            tag_id: other_tag.id, language_code: 'en', primary_flg: true)
-        expect(TagName.where(tag: other_tag, language_code: 'en', primary_flg: true))
+            tag_id: other_tag.id, language_code: 'ain', primary_flg: true)
+        expect(TagName.where(tag: other_tag, language_code: 'ain', primary_flg: true))
             .to contain_exactly(other_primary)
-        expect(primary_tag_name_for(tag.reload, 'en').name).to eq(primary_name_before)
+        expect(primary_tag_name_for(tag.reload, 'ain').name).to eq(primary_name_before)
         expect(tag.category).to eq(category_before)
         aliases_after =
-            tag.tag_names.where(language_code: 'en', primary_flg: false)
+            tag.tag_names.where(language_code: 'ain', primary_flg: false)
                 .order(:name).pluck(:name)
         expect(aliases_after).to eq(aliases_before)
       end
     end
 
     it 'allows a rename to a name already used in Nipponese' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       nipponese = create(:tag, primary_name: 'rename_target')
 
-      patch "/tags/#{ tag.id }", params: { locale: 'en', name: 'rename_target' }
+      patch "/tags/#{ tag.id }", params: { locale: 'ain-Kana', name: 'rename_target' }
 
       expect(response).to have_http_status(:ok), response.body
-      expect(primary_tag_name_for(tag.reload, 'en')).to have_attributes(
-          name: 'rename_target', language_code: 'en', tag_id: tag.id)
+      expect(primary_tag_name_for(tag.reload, 'ain')).to have_attributes(
+          name: 'rename_target', language_code: 'ain', tag_id: tag.id)
       expect(primary_tag_name_for(nipponese.reload, 'ja')).to have_attributes(
           name: 'rename_target', language_code: 'ja', tag_id: nipponese.id)
     end
 
     it 'creates aliases in the requested language' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       put "/tags/#{ tag.id }", params: {
-          locale: 'en', name: english_name.name,
+          locale: 'ain-Kana', name: ainu_name.name,
           category: tag.category, deprecated: false,
-          aliases: 'english_alias', parent_tags: '' }
+          aliases: 'ainu_alias', parent_tags: '' }
 
       expect(response).to have_http_status(:ok), response.body
-      expect(TagName.find_by!(language_code: 'en', name: 'english_alias'))
+      expect(TagName.find_by!(language_code: 'ain', name: 'ainu_alias'))
           .to have_attributes(
-              script_code: 'Latn',
-              tag_id: tag.id, primary_flg: false, language_code: 'en')
-      primary = TagName.find_by!(tag_id: tag.id, language_code: 'en', primary_flg: true)
-      expect(primary).to have_attributes(name: 'english_original', script_code: 'Latn')
+              script_code: 'Kana',
+              tag_id: tag.id, primary_flg: false, language_code: 'ain')
+      primary = TagName.find_by!(tag_id: tag.id, language_code: 'ain', primary_flg: true)
+      expect(primary).to have_attributes(name: 'ainu_original', script_code: 'Kana')
     end
 
     it 'does not take an alias from another language' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       nipponese = create(:tag, primary_name: 'nipponese_owner')
       nipponese_alias = create(:tag_name, :alias, name: 'shared_alias',
@@ -344,7 +327,7 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
           primary_flg: false, language_code: 'ja')
 
       put "/tags/#{ tag.id }", params: {
-          locale: 'en', name: english_name.name,
+          locale: 'ain-Kana', name: ainu_name.name,
           category: tag.category, deprecated: false,
           aliases: 'shared_alias', parent_tags: '' }
 
@@ -352,12 +335,12 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       expect(nipponese_alias.reload).to have_attributes(
           tag_id: nipponese.id,
           primary_flg: false, language_code: 'ja')
-      expect(TagName.find_by!(language_code: 'en', name: 'shared_alias'))
+      expect(TagName.find_by!(language_code: 'ain', name: 'shared_alias'))
           .to have_attributes(
-              tag_id: tag.id, script_code: 'Latn',
-              primary_flg: false, language_code: 'en')
+              tag_id: tag.id, script_code: 'Kana',
+              primary_flg: false, language_code: 'ain')
       expect(TagName.where(name: 'shared_alias').pluck(:language_code, :tag_id))
-          .to contain_exactly(['ja', nipponese.id], ['en', tag.id])
+          .to contain_exactly(['ja', nipponese.id], ['ain', tag.id])
     end
   end
 end

backend/spec/requests/tag_group_locale_spec.rb

@@ -22,7 +22,7 @@ RSpec.describe 'Tag group locale contracts', type: :request do
 
   it 'uses selected-language names in native collection and nested placements' do
     ["/tag_groups/#{ group.id }", '/tag_groups'].each do |path|
-      get path, params: { locale: 'ain' }
+      get path, params: { locale: 'ain-Kana' }
       expect(response).to have_http_status(:ok)
       representation = json.is_a?(Array) ? json.sole : json
       expect(representation.fetch('members').sole.fetch('tag').fetch('name'))
@@ -37,14 +37,14 @@ RSpec.describe 'Tag group locale contracts', type: :request do
   it 'uses snapshot names recursively for history and current names for the latest view' do
     version = group.versions.sole
     ainu_name.update!(name: 'renamed_ainu')
-    get "/wiki/#{ group.id }", params: { locale: 'ain' }
+    get "/wiki/#{ group.id }", params: { locale: 'ain-Kana' }
     expect(response).to have_http_status(:ok)
     expect(json.fetch('body')).to include('renamed_ainu')
-    get "/wiki/#{ group.id }", params: { locale: 'ain', version: version.id }
+    get "/wiki/#{ group.id }", params: { locale: 'ain-Kana', version: version.id }
     expect(response).to have_http_status(:ok)
     expect(json.fetch('body').scan('ainu_member').length).to eq(2)
     expect(json.fetch('body')).not_to include('renamed_ainu', 'japanese_member')
-    get "/wiki/#{ group.id }/diff", params: { locale: 'ain', from: version.id }
+    get "/wiki/#{ group.id }/diff", params: { locale: 'ain-Kana', from: version.id }
     expect(response).to have_http_status(:ok)
     expect(json.fetch('diff').map { _1.fetch('type') }.uniq).to eq(['context'])
   end
@@ -52,23 +52,24 @@ RSpec.describe 'Tag group locale contracts', type: :request do
   ['タググループ:', 'tag_group:', 'TAG_GROUP:'].each do |prefix|
     it "round trips #{ prefix } through create, lookup, search and update" do
       post '/wiki', params: {
-          locale: 'ain', title: "#{ prefix }parent", body: '- tag_group:locale group' },
+          locale: 'ain-Kana', title: "#{ prefix }parent", body: '- tag_group:locale group' },
           headers: headers
       expect(response).to have_http_status(:created)
       id = json.fetch('id')
       expect(json.fetch('body')).to include('タググループ:locale group')
       path = "/wiki/title/#{ CGI.escapeURIComponent("#{ prefix }parent") }"
-      get path, params: { locale: 'ain' }
+      get path, params: { locale: 'ain-Kana' }
       expect(response).to have_http_status(:ok)
       expect(json.fetch('id')).to eq(id)
-      get "#{ path }/exists", params: { locale: 'ain' }
+      get "#{ path }/exists", params: { locale: 'ain-Kana' }
       expect(response).to have_http_status(:no_content)
-      get '/wiki/search', params: { locale: 'ain', title: "#{ prefix }parent" }
+      get '/wiki/search', params: { locale: 'ain-Kana', title: "#{ prefix }parent" }
       expect(json.map { _1.fetch('id') }).to eq([id])
       put "/wiki/#{ id }", params: {
-          locale: 'ain', title: "#{ prefix }renamed", body: '- ainu_member' }, headers: headers
+          locale: 'ain-Kana', title: "#{ prefix }renamed", body: '- ainu_member' },
+          headers: headers
       expect(response).to have_http_status(:ok)
-      get "/tag_groups/#{ id }", params: { locale: 'ain' }
+      get "/tag_groups/#{ id }", params: { locale: 'ain-Kana' }
       expect(json.fetch('members').sole.fetch('tag').fetch('name')).to eq('ainu_member')
     end
   end
@@ -81,7 +82,7 @@ RSpec.describe 'Tag group locale contracts', type: :request do
     end.to raise_error(TagGroupMarkdownParser::InvalidDocument)
     before_state = group_state
     put "/wiki/#{ group.id }", params: {
-        locale: 'ain', title: 'タググループ:partial rename', body: '- japanese_member' },
+        locale: 'ain-Kana', title: 'タググループ:partial rename', body: '- japanese_member' },
         headers: headers
     expect(response).to have_http_status(:unprocessable_entity)
     expect(group_state).to eq(before_state)
@@ -89,15 +90,15 @@ RSpec.describe 'Tag group locale contracts', type: :request do
 
   it 'passes locale into native validation and facade compatibility checks' do
     post '/tag_groups', params: {
-        locale: 'ain', name: 'native', description: nil, members:, placements: },
+        locale: 'ain-Kana', name: 'native', description: nil, members:, placements: },
         headers:, as: :json
     expect(response).to have_http_status(:created)
     id = json.fetch('id')
     expect(json.fetch('members').sole.fetch('tag').fetch('name')).to eq('ainu_member')
-    get "/wiki/#{ id }", params: { locale: 'ain' }
+    get "/wiki/#{ id }", params: { locale: 'ain-Kana' }
     expect(response).to have_http_status(:ok)
     body = json.fetch('body')
-    put "/wiki/#{ id }", params: { locale: 'ain', title: 'tag_group:native', body: },
+    put "/wiki/#{ id }", params: { locale: 'ain-Kana', title: 'tag_group:native', body: },
         headers: headers
     expect(response).to have_http_status(:ok)
     expect(TagGroup.find(id).version_no).to eq(1)
@@ -115,7 +116,7 @@ RSpec.describe 'Tag group locale contracts', type: :request do
     representation = TagGroupRepr.historical(ainu_locale, version)
     expect(representation.fetch('members').sole.fetch('tag').fetch('name')).to be_nil
     expect(representation.fetch('placements').sole.fetch('tag').fetch('name')).to be_nil
-    get "/wiki/#{ old_group.id }", params: { locale: 'ain', version: version.id }
+    get "/wiki/#{ old_group.id }", params: { locale: 'ain-Kana', version: version.id }
     expect(response).to have_http_status(:unprocessable_entity)
     expect(response.body).not_to include('historical_japanese', 'current_ainu')
   end

backend/spec/requests/wiki_locale_spec.rb

@@ -20,12 +20,12 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
 
   ['/wiki', '/wiki/search'].each do |path|
     it "filters #{ path } by the selected language" do
-      get path, params: { locale: 'ain' }
+      get path, params: { locale: 'ain-Kana' }
       expect(response).to have_http_status(:ok)
       expect(json.map { _1.fetch('id') }).to eq([ainu.id])
       expect(json.sole).to include('title' => 'ainu_title', 'body' => 'Ainu body')
 
-      get path, params: { locale: 'ain', title: 'japanese' }
+      get path, params: { locale: 'ain-Kana', title: 'japanese' }
       expect(response).to have_http_status(:ok)
       expect(json).to eq([])
     end
@@ -39,17 +39,17 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
 
   it 'resolves titles and aliases only in the selected language in both APIs' do
     ['/wiki/title/japanese_title', '/tags/name/japanese_title/description'].each do |path|
-      get path, params: { locale: 'ain' }
+      get path, params: { locale: 'ain-Kana' }
       expect(response).to have_http_status(:not_found)
     end
     ['/wiki/title/ainu_title', '/tags/name/ainu_title/description'].each do |path|
-      get path, params: { locale: 'ain' }
+      get path, params: { locale: 'ain-Kana' }
       expect(response).to have_http_status(:ok)
       expect(json.fetch('body')).to eq('Ainu body')
     end
-    get '/wiki/title/ainu_title/exists', params: { locale: 'ain' }
+    get '/wiki/title/ainu_title/exists', params: { locale: 'ain-Kana' }
     expect(response).to have_http_status(:no_content)
-    get '/wiki/title/japanese_title/exists', params: { locale: 'ain' }
+    get '/wiki/title/japanese_title/exists', params: { locale: 'ain-Kana' }
     expect(response).to have_http_status(:not_found)
   end
 
@@ -57,14 +57,14 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
     revision = current_description_version(japanese)
     ["/wiki/#{ japanese.id }", "/wiki/#{ japanese.id }/exists",
      "/wiki/#{ japanese.id }/diff"].each do |path|
-      get path, params: { locale: 'ain', from: revision.id }
+      get path, params: { locale: 'ain-Kana', from: revision.id }
       expect(response).to have_http_status(:not_found)
       expect(response.body).not_to include('Japanese body')
     end
-    get '/wiki/changes', params: { locale: 'ain', id: japanese.id }
+    get '/wiki/changes', params: { locale: 'ain-Kana', id: japanese.id }
     expect(response).to have_http_status(:ok)
     expect(json).to eq([])
-    get '/wiki/changes', params: { locale: 'ain' }
+    get '/wiki/changes', params: { locale: 'ain-Kana' }
     expect(json.map { _1.fetch('revision_id') })
         .to eq([current_description_version(ainu).id])
   end
@@ -73,7 +73,7 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
     snapshot = [japanese.attributes, ainu.attributes, ainu_name.attributes]
     expect do
       put "/wiki/#{ japanese.id }", params: {
-          locale: 'ain', title: 'renamed', body: 'wrong target' }, headers: headers
+          locale: 'ain-Kana', title: 'renamed', body: 'wrong target' }, headers: headers
     end.not_to change(TagDescriptionVersion, :count)
     expect(response).to have_http_status(:not_found)
     expect([japanese.reload.attributes, ainu.reload.attributes, ainu_name.reload.attributes])
@@ -89,10 +89,10 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
       snapshot = original.attributes
       expect do
         if api == :wiki
-          post '/wiki', params: { locale: 'ain', title: 'other_ainu', body: 'created' },
+          post '/wiki', params: { locale: 'ain-Kana', title: 'other_ainu', body: 'created' },
                headers: headers
         else
-          put "/tags/#{ other.id }/description", params: { locale: 'ain', body: 'created' },
+          put "/tags/#{ other.id }/description", params: { locale: 'ain-Kana', body: 'created' },
               headers: headers
         end
       end.to change(TagDescription, :count).by(1)
@@ -108,20 +108,20 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
   it 'shares selected-language edits and revision identity across both APIs' do
     snapshot = japanese.attributes
     first = current_description_version(ainu)
-    put "/tags/#{ tag.id }/description", params: { locale: 'ain', body: 'native edit' },
+    put "/tags/#{ tag.id }/description", params: { locale: 'ain-Kana', body: 'native edit' },
         headers: headers
     expect(response).to have_http_status(:ok)
     second = current_description_version(ainu)
     expect(second).to have_attributes(language_code: 'ain', version_no: 2)
     expect(second.id).not_to eq(first.id)
-    get "/wiki/#{ ainu.id }", params: { locale: 'ain' }
+    get "/wiki/#{ ainu.id }", params: { locale: 'ain-Kana' }
     expect(json).to include('body' => 'native edit', 'revision_id' => second.id)
 
     put "/wiki/#{ ainu.id }", params: {
-        locale: 'ain', title: 'ainu_title', body: 'facade edit',
+        locale: 'ain-Kana', title: 'ainu_title', body: 'facade edit',
         base_revision_id: second.id }, headers: headers
     expect(response).to have_http_status(:ok)
-    get "/tags/#{ tag.id }/description", params: { locale: 'ain' }
+    get "/tags/#{ tag.id }/description", params: { locale: 'ain-Kana' }
     expect(json).to include('body' => 'facade edit', 'language_code' => 'ain', 'version_no' => 3)
     expect(japanese.reload.attributes).to eq(snapshot)
   end
@@ -133,7 +133,7 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
     snapshot = [ainu.reload.attributes, ainu_name.attributes, japanese.attributes]
     expect do
       put "/wiki/#{ ainu.id }", params: {
-          locale: 'ain', title: 'conflicting_name', body: 'current',
+          locale: 'ain-Kana', title: 'conflicting_name', body: 'current',
           base_revision_id: stale.id }, headers: headers
     end.to change(TagDescriptionVersion, :count).by(0).and change(TagVersion, :count).by(0)
     expect(response).to have_http_status(:conflict)
@@ -145,14 +145,14 @@ RSpec.describe 'Wiki and native API locale contracts', type: :request do
     first = current_description_version(ainu)
     TagDescriptionOperation.update!(
         ainu_locale, tag:, body: 'later', updated_by_user: user)
-    get "/wiki/#{ ainu.id }", params: { locale: 'ain', version: first.id }
+    get "/wiki/#{ ainu.id }", params: { locale: 'ain-Kana', version: first.id }
     expect(response).to have_http_status(:ok)
     expect(json).to include('body' => 'Ainu body', 'revision_id' => first.id)
-    get "/wiki/#{ ainu.id }/diff", params: { locale: 'ain', from: first.id }
+    get "/wiki/#{ ainu.id }/diff", params: { locale: 'ain-Kana', from: first.id }
     expect(response).to have_http_status(:ok)
     expect(json.fetch('newer_revision_id')).to eq(current_description_version(ainu).id)
     get "/wiki/#{ ainu.id }/diff", params: {
-        locale: 'ain', from: current_description_version(japanese).id }
+        locale: 'ain-Kana', from: current_description_version(japanese).id }
     expect(response).to have_http_status(:not_found)
   end
 end

backend/spec/services/post_bulk_creator_spec.rb

@@ -1,7 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe PostBulkCreator do
-  include_context 'English locale'
+  include_context 'Ainu locale'
 
   it 'limits workers to two and keeps failures in their request slots' do
     locale = Locale.nipponese
@@ -67,8 +67,7 @@ RSpec.describe PostBulkCreator do
     expect(results[3].dig(:post, :id)).to eq(4)
   end
 
-  it 'passes a future English locale to preflight and creation in every worker' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
+  it 'passes a future Ainu locale to preflight and creation in every worker' do
     locale
     actor = instance_double(User, id: 123)
     allow(User).to receive(:find).with(123).and_return(actor)

backend/spec/services/post_create_plan_spec.rb

@@ -11,23 +11,22 @@ RSpec.describe PostCreatePlan do
   end
 
   context 'with an explicit locale' do
-    include_context 'English locale'
+    include_context 'Ainu locale'
 
     it 'resolves the matching language identity and its parents' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
-      english_name = create(:tag_name, name: 'shared_name',
-                                      language_code: 'en', script_code: 'Latn')
-      english = create(:tag, primary_tag_name: english_name, category: :character)
+      ainu_name = create(:tag_name, name: 'shared_name',
+                                      language_code: 'ain', script_code: 'Kana')
+      ainu = create(:tag, primary_tag_name: ainu_name, category: :character)
       nipponese = create(:tag, primary_name: 'temporary_nipponese', category: :general)
       # The DB permits this identity; validation has its own contract spec.
       primary_tag_name_for(nipponese, 'ja').update_columns(name: 'shared_name')
-      english_parent_name = create(:tag_name, name: 'english_parent',
-                                            language_code: 'en', script_code: 'Latn')
-      english_parent = create(:tag, primary_tag_name: english_parent_name,
+      ainu_parent_name = create(:tag_name, name: 'ainu_parent',
+                                            language_code: 'ain', script_code: 'Kana')
+      ainu_parent = create(:tag, primary_tag_name: ainu_parent_name,
                                     category: :material)
       nipponese_parent = create(:tag, primary_name: 'nipponese_parent', category: :general)
-      TagImplication.create!(tag: english, parent_tag: english_parent)
+      TagImplication.create!(tag: ainu, parent_tag: ainu_parent)
       TagImplication.create!(tag: nipponese, parent_tag: nipponese_parent)
 
       plan = described_class.new(attributes: { tags: 'shared_name' }).build!(locale)
@@ -35,35 +34,34 @@ RSpec.describe PostCreatePlan do
       expect(plan[:direct_tag_specs]).to eq(
           [{ name: 'shared_name', category: :character }])
       expect(plan[:post_tag_specs]).to include(
-          { name: primary_tag_name_for(english_parent, 'en').name, category: :material })
+          { name: primary_tag_name_for(ainu_parent, 'ain').name, category: :material })
       expect(plan[:post_tag_specs].pluck(:name))
           .not_to include(primary_tag_name_for(nipponese_parent, 'ja').name)
     end
 
     it 'resolves aliases within the requested language' do
-      pending '日本語以外の Locale 有効化は別課題で実装する'
       locale
       nipponese = create(:tag, primary_name: 'nipponese_canonical')
-      english_name = create(:tag_name, name: 'english_canonical',
-                                      language_code: 'en', script_code: 'Latn')
-      english = create(:tag, primary_tag_name: english_name, category: :character)
+      ainu_name = create(:tag_name, name: 'ainu_canonical',
+                                      language_code: 'ain', script_code: 'Kana')
+      ainu = create(:tag, primary_tag_name: ainu_name, category: :character)
       nipponese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                   tag: nipponese)
-      english_alias = create(:tag_name, :alias, name: 'shared_alias',
-                                               tag: english,
-                                               language_code: 'en', script_code: 'Latn')
+      ainu_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                               tag: ainu,
+                                               language_code: 'ain', script_code: 'Kana')
 
       expect(nipponese_alias.reload).to have_attributes(
           tag_id: nipponese.id,
           primary_flg: false, language_code: 'ja')
-      expect(english_alias.reload).to have_attributes(
-          tag_id: english.id,
-          primary_flg: false, language_code: 'en')
+      expect(ainu_alias.reload).to have_attributes(
+          tag_id: ainu.id,
+          primary_flg: false, language_code: 'ain')
 
       plan = described_class.new(attributes: { tags: 'shared_alias' }).build!(locale)
 
       expect(plan[:direct_tag_specs]).to eq(
-          [{ name: english_name.name, category: :character }])
+          [{ name: ainu_name.name, category: :character }])
     end
   end
 

backend/spec/services/post_creator_spec.rb

@@ -77,11 +77,10 @@ RSpec.describe PostCreator do
   end
 
   context 'with an explicit locale' do
-    include_context 'English locale'
+    include_context 'Ainu locale'
 
     [false, true].each do |planned|
       it "passes locale to tag creation with planned attributes: #{ planned }" do
-        pending '日本語以外の Locale 有効化は別課題で実装する'
         locale
         attributes = {
             title: 'Locale propagation',
@@ -97,8 +96,8 @@ RSpec.describe PostCreator do
 
         expect(Tag).to have_received(:find_or_create_by_tag_name!)
             .with(locale, 'locale_character', category: :character).at_least(:once)
-        tag_name = TagName.find_by!(language_code: 'en', name: 'locale_character')
-        expect(tag_name).to have_attributes(script_code: 'Latn', primary_flg: true)
+        tag_name = TagName.find_by!(language_code: 'ain', name: 'locale_character')
+        expect(tag_name).to have_attributes(script_code: 'Kana', primary_flg: true)
         expect(post.tags).to include(tag_name.tag)
       end
     end

backend/spec/services/tag_name_completion_spec.rb

@@ -0,0 +1,68 @@
+require 'rails_helper'
+
+RSpec.describe TagNameCompletion do
+  let!(:language) { Language.create!(code: 'ain', name: 'ain') }
+  let!(:script) { Script.create!(code: 'Kana', name: 'Kana') }
+  let(:locale) do
+    Locale.create!(code: 'ain-Kana', language_code: 'ain', script_code: 'Kana',
+                   region_code: 392, variants: '', name: 'ain-Kana')
+  end
+
+  it 'generates a phonetic primary with the required identity and provenance' do
+    tag = create(:tag, primary_name: '漢字あカ')
+
+    locale
+
+    expect(tag.reload.tag_name('ain')).to have_attributes(
+        name: 'カンチ_ア_カ', language_code: 'ain', script_code: 'Kana',
+        primary_flg: true, auto_generated: true, tag_id: tag.id)
+  end
+
+  it 'uses the exact fallback when the input cannot be converted' do
+    tag = create(:tag, primary_name: '😀')
+
+    locale
+
+    expect(tag.reload.name('ain')).to eq("Tag_##{ tag.id }")
+  end
+
+  it 'uses the exact fallback when a generated name collides with a human name' do
+    other = create(:tag, primary_name: 'other')
+    create(:tag_name, tag: other, name: 'AbC', language_code: 'ain', script_code: 'Kana')
+    tag = create(:tag, primary_name: 'AbC')
+
+    locale
+
+    expect(tag.reload.name('ain')).to eq("Tag_##{ tag.id }")
+    expect(other.reload.name('ain')).to eq('AbC')
+  end
+
+  it 'uses the exact fallback for a transcription that fails name validation' do
+    tag = create(:tag, primary_name: 'base')
+    source = TagName.new(name: 'nico:invalid', language_code: 'ja')
+
+    expect(TagName.generate_name(locale, tag, source)).to eq("Tag_##{ tag.id }")
+  end
+
+  it 'preserves human confirmation and is idempotent across repeated completion' do
+    locale
+    tag = Tag.find_or_create_by_tag_name!(Locale.nipponese, 'あいぬ', category: :general)
+    primary = tag.tag_name('ain')
+    primary.update!(name: 'human_confirmed', auto_generated: false)
+    snapshot = tag.tag_names.order(:id).map(&:attributes)
+
+    2.times { described_class.for_tag!(tag) }
+
+    expect(tag.tag_names.order(:id).map(&:attributes)).to eq(snapshot)
+  end
+
+  it 'creates one primary per active language through the production creation path' do
+    locale
+
+    tag = Tag.find_or_create_by_tag_name!(Locale.nipponese, 'アイヌ', category: :general)
+
+    expect(tag.tag_names.where(primary_flg: true).pluck(:language_code, :name))
+        .to contain_exactly(['ja', 'アイヌ'], ['ain', 'アイヌ'])
+    expect(Tag.find_or_create_by_tag_name!(locale, 'アイヌ', category: :general)).to eq(tag)
+  end
+end

backend/spec/services/tag_name_transcription_spec.rb

@@ -0,0 +1,34 @@
+require 'rails_helper'
+
+RSpec.describe TagNameTranscription do
+  {
+      'AbCdXYZ' => 'AbCdXYZ',
+      'あいぬ' => 'アイヌ',
+      'アイヌ' => 'アイヌ',
+      '日本語' => 'ニチホンコ',
+      '漢字' => 'カンチ',
+      '漢あカ' => 'カン_ア_カ',
+      '漢字あいうカナ' => 'カンチ_アイウ_カナ',
+      'Ab漢あカZ' => 'Abカン_ア_カZ',
+      'りゃリュリョ' => 'リヤ_リユリヨ',
+      'キャット' => 'キヤット',
+      'コーヒー' => 'コヒ',
+      'コウコオケイケエカア' => 'コウコオケイケエカア',
+      'ガギグゲゴダヂヅデド' => 'カキクケコタチチュテト',
+      'バビブベボパピプペポ' => 'パピプペポパピプペポ',
+      'ザジズゼゾ' => 'チャチチュチェチョ',
+      'ジャジュジョ' => 'チヤチユチヨ',
+      'アㇰアㇱアッアンアㇺアㇷ゚' => 'アㇰアㇱアッアンアㇺアㇷ゚',
+      'アㇻイㇼウㇽエㇾオㇿ' => 'アㇻイㇼウㇽエㇾオㇿ',
+      '漢Aあ' => 'カンAア' }.each do |source, expected|
+    it "transcribes #{ source } as #{ expected }" do
+      expect(described_class.ainu(source)).to eq(expected)
+    end
+  end
+
+  ['😀', 'ヴ', 'ティ', 'ㇵ', '魑', '𠮷', 'ー', '々'].each do |source|
+    it "rejects unsupported input #{ source }" do
+      expect { described_class.ainu(source) }.to raise_error(described_class::Unconvertible)
+    end
+  end
+end

backend/spec/services/tag_version_recorder_spec.rb

@@ -1,7 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe TagVersionRecorder do
-  include_context 'English locale'
+  include_context 'Ainu locale'
 
   let(:user) { create_member_user! }
   let!(:tag) { create(:tag, primary_name: '日本語名') }
@@ -17,11 +17,11 @@ RSpec.describe TagVersionRecorder do
                     { 'name' => 'alias_z', 'script_code' => 'Jpan' }] }
   end
 
-  def prepare_english_names!
+  def prepare_ainu_names!
     locale
-    primary_tag_name_for(tag, 'en').update!(name: 'English name', auto_generated: false)
-    create(:tag_name, :alias, tag:, language_code: 'en', script_code: 'Latn',
-                              name: 'English alias')
+    primary_tag_name_for(tag, 'ain').update!(name: 'Ainu name', auto_generated: false)
+    create(:tag_name, :alias, tag:, language_code: 'ain', script_code: 'Kana',
+                              name: 'Ainu alias')
   end
 
   it 'snapshots the Nipponese primary and sorted aliases in grouped names' do
@@ -62,35 +62,32 @@ RSpec.describe TagVersionRecorder do
   end
 
   it 'snapshots future languages together without a request locale argument' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_names!
+    prepare_ainu_names!
 
     version = described_class.record!(tag:, event_type: :create, created_by_user: user)
 
     expect(version.names).to eq(
         'ja' => nipponese_snapshot,
-        'en' => { 'name' => 'English name', 'script_code' => 'Latn',
+        'ain' => { 'name' => 'Ainu name', 'script_code' => 'Kana',
                   'auto_generated' => false,
-                  'aliases' => [{ 'name' => 'English alias', 'script_code' => 'Latn' }] })
+                  'aliases' => [{ 'name' => 'Ainu alias', 'script_code' => 'Kana' }] })
   end
 
-  it 'creates a version for an English-only change and preserves prior names' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_names!
+  it 'creates a version for an Ainu-only change and preserves prior names' do
+    prepare_ainu_names!
     original = described_class.record!(tag:, event_type: :create, created_by_user: user)
-    primary_tag_name_for(tag, 'en').update!(name: 'Renamed')
+    primary_tag_name_for(tag, 'ain').update!(name: 'Renamed')
 
     updated = described_class.record!(tag:, event_type: :update, created_by_user: user)
 
     expect(updated.version_no).to eq(2)
-    expect(original.reload.names.fetch('en')).to include('name' => 'English name')
-    expect(updated.names.fetch('en')).to include('name' => 'Renamed')
+    expect(original.reload.names.fetch('ain')).to include('name' => 'Ainu name')
+    expect(updated.names.fetch('ain')).to include('name' => 'Renamed')
     expect(updated.names.fetch('ja')).to eq(nipponese_snapshot)
   end
 
   it 'does not create a version for an unchanged multilingual snapshot' do
-    pending '日本語以外の Locale 有効化は別課題で実装する'
-    prepare_english_names!
+    prepare_ainu_names!
     described_class.record!(tag:, event_type: :create, created_by_user: user)
 
     unchanged = described_class.record!(tag:, event_type: :update, created_by_user: user)

backend/spec/support/ainu_locale_contract.rb

@@ -1,19 +1,22 @@
 RSpec.shared_context 'Ainu locale contract' do
-  let(:ainu_locale) { instance_double(Locale, language_code: 'ain', script_code: 'Jpan') }
+  let(:ainu_locale) do
+    instance_double(Locale, language_code: 'ain', script_code: 'Kana', deprecated_at: nil)
+  end
 
   before do
-    # Only the foreign-key reference is seeded; no Locale or enum is enabled.
+    # Keep description contracts independent of locale activation and backfill.
     Language.connection.execute(<<~SQL)
       INSERT IGNORE INTO languages (code, name, created_at)
       VALUES ('ain', 'Ainu', CURRENT_TIMESTAMP)
     SQL
+    Script.find_or_create_by!(code: 'Kana') { _1.name = 'Kana' }
     allow(Locale).to receive(:find_by).and_call_original
-    allow(Locale).to receive(:find_by).with(code: 'ain').and_return(ainu_locale)
+    allow(Locale).to receive(:find_by).with(code: 'ain-Kana').and_return(ainu_locale)
   end
 
   def add_ainu_name tag, name
     TagName.insert_all!([{
-        tag_id: tag.id, language_code: 'ain', script_code: 'Jpan', name:,
+        tag_id: tag.id, language_code: 'ain', script_code: 'Kana', name:,
         primary_flg: true, auto_generated: false,
         created_at: Time.current, updated_at: Time.current }])
     tag.tag_names.reset

backend/spec/support/locale_context.rb

@@ -1,22 +1,17 @@
-RSpec.shared_context 'English locale' do
-  def prepare_english_reference!
-    Language.find_or_create_by!(code: 'en') { _1.name = 'English' }
-    Script.find_or_create_by!(code: 'Latn') { _1.name = 'Latin' }
-    region_class = Class.new(ApplicationRecord) { self.table_name = 'regions' }
-    region_class.find_or_create_by!(code: 826) do |record|
-      record.alpha_code = 'GB'
-      record.name = 'United Kingdom'
-    end
+RSpec.shared_context 'Ainu locale' do
+  def prepare_ainu_reference!
+    Language.find_or_create_by!(code: 'ain') { _1.name = 'ain' }
+    Script.find_or_create_by!(code: 'Kana') { _1.name = 'Kana' }
   end
 
   let(:locale) do
-    prepare_english_reference!
-    Locale.find_or_create_by!(code: 'en') do |record|
-      record.language_code = 'en'
-      record.script_code = 'Latn'
-      record.region_code = 826
+    prepare_ainu_reference!
+    Locale.find_or_create_by!(code: 'ain-Kana') do |record|
+      record.language_code = 'ain'
+      record.script_code = 'Kana'
+      record.region_code = 392
       record.variants = ''
-      record.name = 'English'
+      record.name = 'Ainu'
     end
   end
 end

frontend/src/App.tsx

@@ -5,7 +5,8 @@ import { createBrowserRouter,
 	 Route,
 	 RouterProvider,
 	 Routes,
-	 useLocation } from 'react-router-dom'
+	 useLocation,
+	 useParams } from 'react-router-dom'
 
 import DevModeWatermark from '@/components/DevModeWatermark'
 import RouteBlockerOverlay from '@/components/RouteBlockerOverlay'
@@ -13,6 +14,8 @@ import TopNav from '@/components/TopNav'
 import DialogueProvider from '@/components/dialogues/DialogueProvider'
 import { Toaster } from '@/components/ui/toaster'
 import { apiPost, isApiError } from '@/lib/api'
+import { isLocaleCode, legacyLocalePath, switchLocalePath } from '@/lib/locales'
+import { useLocale } from '@/lib/useLocale'
 import { applyClientAnimationMode,
 	 applyClientAppearance,
 	 fetchUserThemeSlots,
@@ -73,41 +76,41 @@ const RouteTransitionWrapper = ({ animationMode, user, setUser }: {
 
   const routes = (
       <Routes location={location}>
-	<Route path="/" element={<Navigate to="/posts" replace/>}/>
-	<Route path="/posts" element={<PostListPage/>}/>
-	<Route path="/posts/new" element={<PostNewPage user={user}/>}/>
-	<Route path="/posts/search" element={<PostSearchPage/>}/>
-	<Route path="/posts/:id" element={<PostDetailRoute user={user}/>}/>
-	<Route path="/posts/changes" element={<PostHistoryPage/>}/>
-	<Route path="/tags" element={<TagListPage/>}/>
-	<Route path="/tags/:id" element={<TagDetailPage/>}/>
-	<Route path="/tags/:id/deerjikists" element={<DeerjikistDetailPage/>}/>
-	<Route path="/tags/nico" element={<NicoTagListPage user={user}/>}/>
-	<Route path="/nico/tags" element={<NicoTagListPage user={user}/>}/>
-	<Route path="/tags/changes" element={<TagHistoryPage/>}/>
-	<Route path="/theatres/:id" element={<TheatreDetailPage user={user}/>}/>
-	<Route path="/materials" element={<MaterialBasePage/>}>
+	<Route path="" element={<LocaleRedirect to="/posts"/>}/>
+	<Route path="posts" element={<PostListPage/>}/>
+	<Route path="posts/new" element={<PostNewPage user={user}/>}/>
+	<Route path="posts/search" element={<PostSearchPage/>}/>
+	<Route path="posts/:id" element={<PostDetailRoute user={user}/>}/>
+	<Route path="posts/changes" element={<PostHistoryPage/>}/>
+	<Route path="tags" element={<TagListPage/>}/>
+	<Route path="tags/:id" element={<TagDetailPage/>}/>
+	<Route path="tags/:id/deerjikists" element={<DeerjikistDetailPage/>}/>
+	<Route path="tags/nico" element={<NicoTagListPage user={user}/>}/>
+	<Route path="nico/tags" element={<NicoTagListPage user={user}/>}/>
+	<Route path="tags/changes" element={<TagHistoryPage/>}/>
+	<Route path="theatres/:id" element={<TheatreDetailPage user={user}/>}/>
+	<Route path="materials" element={<MaterialBasePage/>}>
 	  <Route index element={<MaterialListPage/>}/>
 	  <Route path="changes" element={<MaterialHistoryPage/>}/>
 	  <Route path="new" element={<MaterialNewPage/>}/>
 	  <Route path="suppressions" element={<MaterialSyncSuppressionsPage/>}/>
 	  <Route path=":id" element ={<MaterialDetailPage/>}/>
 	</Route>
-	<Route path="/wiki" element={<WikiSearchPage/>}/>
-	<Route path="/wiki/:title" element={<WikiDetailPage/>}/>
-	<Route path="/wiki/new" element={<WikiNewPage user={user}/>}/>
-	<Route path="/wiki/:id/edit" element={<WikiEditPage user={user}/>}/>
-	<Route path="/wiki/:id/diff" element={<WikiDiffPage/>}/>
-	<Route path="/wiki/changes" element={<WikiHistoryPage/>}/>
-	<Route path="/help" element={<HelpListPage/>}/>
-	<Route path="/help/changes" element={<HelpChangesPage/>}/>
-	<Route path="/help/:identifier/diff" element={<HelpDiffPage/>}/>
-	<Route path="/help/:identifier" element={<HelpDetailPage/>}/>
-	<Route path="/users/settings" element={<SettingPage user={user} setUser={setUser}/>}/>
-	<Route path="/settings" element={<Navigate to="/users/settings" replace/>}/>
-	<Route path="/tos" element={<TOSPage/>}/>
-	<Route path="/gekanator" element={<GekanatorPage user={user}/>}/>
-	<Route path="/more" element={<MorePage/>}/>
+	<Route path="wiki" element={<WikiSearchPage/>}/>
+	<Route path="wiki/:title" element={<WikiDetailPage/>}/>
+	<Route path="wiki/new" element={<WikiNewPage user={user}/>}/>
+	<Route path="wiki/:id/edit" element={<WikiEditPage user={user}/>}/>
+	<Route path="wiki/:id/diff" element={<WikiDiffPage/>}/>
+	<Route path="wiki/changes" element={<WikiHistoryPage/>}/>
+	<Route path="help" element={<HelpListPage/>}/>
+	<Route path="help/changes" element={<HelpChangesPage/>}/>
+	<Route path="help/:identifier/diff" element={<HelpDiffPage/>}/>
+	<Route path="help/:identifier" element={<HelpDetailPage/>}/>
+	<Route path="users/settings" element={<SettingPage user={user} setUser={setUser}/>}/>
+	<Route path="settings" element={<LocaleRedirect to="/users/settings"/>}/>
+	<Route path="tos" element={<TOSPage/>}/>
+	<Route path="gekanator" element={<GekanatorPage user={user}/>}/>
+	<Route path="more" element={<MorePage/>}/>
 	<Route path="*" element={<NotFound/>}/>
       </Routes>)
 
@@ -259,9 +262,27 @@ const RoutedApp: FC = () => {
 }
 
 
-const router = createBrowserRouter ([{
-    path: '*',
-    element: <RoutedApp/> }])
+const LocaleRoute: FC = () => {
+  const { locale } = useParams ()
+  const location = useLocation ()
+  if (isLocaleCode (locale))
+    return <RoutedApp/>
+
+  const redirect = legacyLocalePath (location)
+  return redirect == null ? <NotFound/> : <Navigate to={redirect} replace/>
+}
+
+
+const LocaleRedirect = ({ to }: { to: string }) => {
+  const locale = useLocale ()
+  const location = useLocation ()
+  return <Navigate to={switchLocalePath ({ ...location, pathname: to }, locale)} replace/>
+}
+
+
+const router = createBrowserRouter ([
+    { path: '/:locale/*', element: <LocaleRoute/> },
+    { path: '*', element: <LocaleRoute/> }])
 
 
 const App: FC = () => (

frontend/src/components/MaterialSidebar.tsx

@@ -1,7 +1,8 @@
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useRef, useState } from 'react'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import SidebarComponent from '@/components/layout/SidebarComponent'
 import TagLink from '@/components/TagLink'
 import { fetchMaterialTagTree, parseMaterialFilter } from '@/lib/materials'
@@ -83,7 +84,7 @@ const tagSelectionShellClass = (selected: boolean): string =>
 const updateMaterialFilterQuery = (
     pathname: string,
     locationSearch: string,
-    navigate: ReturnType<typeof useNavigate>,
+    navigate: ReturnType<typeof useLocaleNavigate>,
     materialFilter: MaterialFilter,
 ) => {
   const qs = new URLSearchParams (locationSearch)
@@ -125,10 +126,11 @@ const MaterialTreeNode: FC<{
     tag:            MaterialSidebarTag
 }> = ({ materialFilter, nestLevel = 0, onChildren, openTags, selectedTagId,
 	 setOpenTags, tag }) => {
+  const locale = useLocale ()
   const open = Boolean (openTags[tag.id])
   const { data } = useQuery ({
-      queryKey: materialsKeys.tree ({ parentId: tag.id, materialFilter }),
-      queryFn:	() => fetchMaterialTagTree ({ parentId: tag.id, materialFilter }),
+      queryKey: materialsKeys.tree ({ parentId: tag.id, materialFilter }, locale),
+      queryFn:	() => fetchMaterialTagTree ({ parentId: tag.id, materialFilter }, locale),
       enabled:	open && tag.hasChildren && tag.children.length === 0})
 
   useEffect (() => {
@@ -200,6 +202,7 @@ const MobileMaterialTreeNode: FC<{ depth?: number
       tag,
     },
 ) => {
+  const locale = useLocale ()
   const open = Boolean (openTags[tag.id])
   const tagColumnRef = useRef<HTMLDivElement | null> (null)
   const chipRef = useRef<HTMLDivElement | null> (null)
@@ -210,8 +213,8 @@ const MobileMaterialTreeNode: FC<{ depth?: number
   const [tagLinkInlineSizePx, setTagLinkInlineSizePx] = useState<number | null> (null)
   const [childAvailableInlineSizePx, setChildAvailableInlineSizePx] = useState<number | null> (null)
   const { data } = useQuery ({
-      queryKey: materialsKeys.tree ({ parentId: tag.id, materialFilter }),
-      queryFn:  () => fetchMaterialTagTree ({ parentId: tag.id, materialFilter }),
+      queryKey: materialsKeys.tree ({ parentId: tag.id, materialFilter }, locale),
+      queryFn:  () => fetchMaterialTagTree ({ parentId: tag.id, materialFilter }, locale),
       enabled:  open && tag.hasChildren && tag.children.length === 0})
 
   useEffect (() => {
@@ -382,8 +385,9 @@ const MobileMaterialTreeNode: FC<{ depth?: number
 
 
 const MaterialSidebar: FC = () => {
+  const locale = useLocale ()
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const qs = new URLSearchParams (location.search)
   const materialFilter = parseMaterialFilter (qs.get ('material_filter'), 'any')
   const selectedTagId = Number (qs.get ('tag_id') ?? 0) || null
@@ -395,8 +399,8 @@ const MaterialSidebar: FC = () => {
       useState<number | null> (null)
 
   const { data: rootTags = [], isLoading, isError } = useQuery ({
-      queryKey: materialsKeys.tree ({ parentId: null, materialFilter }),
-      queryFn:  () => fetchMaterialTagTree ({ parentId: null, materialFilter })})
+      queryKey: materialsKeys.tree ({ parentId: null, materialFilter }, locale),
+      queryFn:  () => fetchMaterialTagTree ({ parentId: null, materialFilter }, locale)})
 
   useEffect (() => {
     setDesktopTags (rootTags)

frontend/src/components/PostEditForm.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useMemo, useState } from 'react'
 
+import { useLocale } from '@/lib/useLocale'
 import PostOriginalCreatedTimeField from '@/components/PostOriginalCreatedTimeField'
 import FieldError from '@/components/common/FieldError'
 import PostDurationField from '@/components/posts/PostDurationField'
@@ -51,6 +52,7 @@ type Props = { post:    Post
 
 
 const PostEditForm: FC<Props> = ({ post, onSave }) => {
+  const locale = useLocale ()
   const [disabled, setDisabled] = useState (false)
   const [duration, setDuration] = useState<string> (videoMsToDurationValue (post.videoMs))
   const { baseErrors, fieldErrors, clearValidationErrors, applyValidationError } =
@@ -75,7 +77,7 @@ const PostEditForm: FC<Props> = ({ post, onSave }) => {
 
     try
     {
-      const data = await updatePost (...args)
+      const data = await updatePost (...args, undefined, locale)
       onSave ({ ...post,
 		versionNo:              data.versionNo,
 		title:                  data.title,

frontend/src/components/PostFormTagsArea.tsx

@@ -2,6 +2,7 @@
 
 import { useRef, useState } from 'react'
 
+import { useLocale } from '@/lib/useLocale'
 import TagSearchBox from '@/components/TagSearchBox'
 import FormField from '@/components/common/FormField'
 import TextArea from '@/components/common/TextArea'
@@ -38,6 +39,7 @@ type Props = Omit<ComponentPropsWithoutRef<'textarea'>, 'value' | 'onChange' | '
 
 
 const PostFormTagsArea: FC<Props> = ({ tags, setTags, errors, ...rest }) => {
+  const locale = useLocale ()
   const ref = useRef<HTMLTextAreaElement> (null)
 
   const [bounds, setBounds] = useState<{ start: number; end: number }> ({ start: 0, end: 0 })
@@ -68,7 +70,7 @@ const PostFormTagsArea: FC<Props> = ({ tags, setTags, errors, ...rest }) => {
 
     setBounds ({ start, end })
 
-    const data = await apiGet<Tag[]> ('/tags/autocomplete', { params: {
+    const data = await apiGet<Tag[]> ('/tags/autocomplete', { locale, params: {
 	q: token,
 	nico: '0' } })
     const nextSuggestions = data.filter (t => t.postCount > 0)

frontend/src/components/PrefetchLink.tsx

@@ -1,8 +1,10 @@
 import { useQueryClient } from '@tanstack/react-query'
 import { forwardRef, useMemo } from 'react'
 import { flushSync } from 'react-dom'
-import { createPath, useNavigate } from 'react-router-dom'
+import { createPath } from 'react-router-dom'
 
+import { localiseTo } from '@/lib/locales'
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import { useOverlayStore } from '@/components/RouteBlockerOverlay'
 import { prefetchForURL } from '@/lib/prefetchers'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
@@ -32,13 +34,17 @@ export default forwardRef<HTMLAnchorElement, Props> (({
   if ('onClick' in rest)
     delete rest['onClick']
 
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
+  const locale = useLocale ()
   const qc = useQueryClient ()
   const behaviourSettings = useClientBehaviourSettings ()
   const linkPreloadMode = behaviourSettings.linkPreload ?? 'intent'
   const path = useMemo (
-      () => typeof to === 'string' ? to : createPath (to),
-      [to],
+      () => {
+	const localised = localiseTo (to, locale)
+	return typeof localised === 'string' ? localised : createPath (localised)
+      },
+      [to, locale],
   )
   const url = useMemo (() => {
     return (new URL (path, window.location.origin)).toString ()
@@ -98,7 +104,7 @@ export default forwardRef<HTMLAnchorElement, Props> (({
       if (!(ok) && cancelOnError)
 	return
 
-      navigate (to, { replace, ...(state && { state }) })
+      navigate (path, { replace, ...(state && { state }) })
     }
     catch (ex)
     {

frontend/src/components/TagDetailSidebar.tsx

@@ -12,6 +12,7 @@ import { useQueryClient } from '@tanstack/react-query'
 import { motion } from 'framer-motion'
 import { useEffect, useMemo, useRef, useState } from 'react'
 
+import { useLocale } from '@/lib/useLocale'
 import DraggableDroppableTagRow from '@/components/DraggableDroppableTagRow'
 import PrefetchLink from '@/components/PrefetchLink'
 import TagLink from '@/components/TagLink'
@@ -27,6 +28,7 @@ import { postsKeys, tagsKeys } from '@/lib/queryKeys'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
 import { dateString, originalCreatedAtString } from '@/lib/utils'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { CollisionDetection, DragEndEvent } from '@dnd-kit/core'
 import type { FC, MutableRefObject, ReactNode } from 'react'
 
@@ -180,8 +182,10 @@ const buildFlatTagByCategory = (
 
 const changeCategory = async (
     tagId:    number,
-    category: Category): Promise<void> => {
-  await apiPatch (`/tags/${ tagId }`, { category })
+    category: Category,
+    locale: LocaleCode = 'ja',
+): Promise<void> => {
+  await apiPatch (`/tags/${ tagId }`, { category }, { locale })
 }
 
 
@@ -227,6 +231,7 @@ type ActiveTagDrag = {
 
 
 const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
+  const locale = useLocale ()
   sp = Boolean (sp)
 
   const qc = useQueryClient ()
@@ -271,7 +276,7 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
       useSensor (TouchSensor, { activationConstraint: { delay: 250, tolerance: 8 } }))
 
   const reloadTags = async (): Promise<void> => {
-    setTags (buildTagByCategory (await apiGet<Post> (`/posts/${ post.id }`)))
+    setTags (buildTagByCategory (await apiGet<Post> (`/posts/${ post.id }`, { locale })))
     qc.invalidateQueries ({ queryKey: postsKeys.root })
     qc.invalidateQueries ({ queryKey: tagsKeys.root })
   }
@@ -314,9 +319,9 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 	    return
 
 	  if (fromParentId != null)
-	    await apiDelete (`/tags/${ fromParentId }/children/${ childId }`)
+	    await apiDelete (`/tags/${ fromParentId }/children/${ childId }`, { locale })
 
-	  await apiPost (`/tags/${ parentId }/children/${ childId }`, { })
+	  await apiPost (`/tags/${ parentId }/children/${ childId }`, { }, { locale })
 
 	  await reloadTags ()
 	  toast ({
@@ -333,10 +338,10 @@ const TagDetailSidebar: FC<Props> = ({ className, post, sp }) => {
 	    return
 
 	  if (child.category !== cat)
-	    await changeCategory (childId, cat)
+	    await changeCategory (childId, cat, locale)
 
 	  if (fromParentId != null)
-	    await apiDelete (`/tags/${ fromParentId }/children/${ childId }`)
+	    await apiDelete (`/tags/${ fromParentId }/children/${ childId }`, { locale })
 
 	  const fromParent = fromParentId == null ? null : findTag (tags, fromParentId)
 

frontend/src/components/TagSearch.tsx

@@ -1,8 +1,9 @@
 // TODO: タグ入力系すべてに同様の処理あるため共通化する．
 
 import { useEffect, useState } from 'react'
-import { useNavigate, useLocation } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import { apiGet } from '@/lib/api'
 import { inputClass } from '@/lib/utils'
 
@@ -14,8 +15,9 @@ import type { Tag } from '@/types'
 
 
 const TagSearch: FC = () => {
+  const locale = useLocale ()
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const [activeIndex, setActiveIndex] = useState (-1)
   const [search, setSearch] = useState ('')
@@ -32,7 +34,7 @@ const TagSearch: FC = () => {
 	return
       }
 
-    const data = await apiGet<Tag[]> ('/tags/autocomplete', { params: { q } })
+    const data = await apiGet<Tag[]> ('/tags/autocomplete', { locale, params: { q } })
     setSuggestions (data.filter (t => t.postCount > 0))
     if (suggestions.length > 0)
       setSuggestionsVsbl (true)

frontend/src/components/TagSidebar.tsx

@@ -1,7 +1,8 @@
 import { AnimatePresence, motion } from 'framer-motion'
 import { useEffect, useState } from 'react'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import TagLink from '@/components/TagLink'
 import TagSearch from '@/components/TagSearch'
 import SectionTitle from '@/components/common/SectionTitle'
@@ -22,7 +23,8 @@ type Props = { posts:     Post[]
 
 
 const TagSidebar: FC<Props> = ({ posts, onClick }) => {
-  const navigate = useNavigate ()
+  const locale = useLocale ()
+  const navigate = useLocaleNavigate ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const animationsOff = animationMode === 'off'
@@ -96,7 +98,7 @@ const TagSidebar: FC<Props> = ({ posts, onClick }) => {
 	       try
 	       {
 		 const data = await apiGet<Post> ('/posts/random',
-		     { params: { tags: tagsQuery.split (' ').filter (e => e !== '').join (' '),
+		     { locale, params: { tags: tagsQuery.split (' ').filter (e => e !== '').join (' '),
 				 match: (anyFlg ? 'any' : 'all') } })
 		 navigate (`/posts/${ data.id }`)
 	       }

frontend/src/components/TopNav.tsx

@@ -3,10 +3,18 @@ import { AnimatePresence, motion } from 'framer-motion'
 import { Fragment, useEffect, useLayoutEffect, useRef, useState } from 'react'
 import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import Separator from '@/components/MenuSeparator'
 import PrefetchLink from '@/components/PrefetchLink'
 import TopNavUser from '@/components/TopNavUser'
 import { WikiIdBus } from '@/lib/eventBus/WikiIdBus'
+import { LOCALE_LABELS,
+	 SUPPORTED_LOCALES,
+	 isLocaleCode,
+	 localeFromPath,
+	 switchLocalePath,
+	 unlocalisedPath } from '@/lib/locales'
+import { uiMessage, uiMessageLanguage } from '@/lib/messages'
 import { materialsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
 import { canEditContent } from '@/lib/users'
@@ -24,13 +32,16 @@ type NavDensity = 'auto' | 'comfortable' | 'compact'
 
 
 export const menuOutline = (
-    { tag, material, wikiId, user, pathName }: {
+    { tag, material, wikiId, user, pathName: fullPathname }: {
 	tag?: Tag | null
 	material?: Material | null
 	wikiId: number | null
 	user: User | null
 	pathName: string },
 ): Menu => {
+  const pathName = unlocalisedPath (fullPathname)
+  const locale = localeFromPath (fullPathname) ?? 'ja'
+  const message = (key: Parameters<typeof uiMessage>[1]) => uiMessage (locale, key)
   const postCount = tag?.postCount ?? material?.tag?.postCount ?? 0
   const editable = canEditContent (user)
 
@@ -41,71 +52,73 @@ export const menuOutline = (
 
   const materialFlg = /^\/materials\/\d+/.test (pathName)
 
-  const helpVsbl = /\/help\/((?!changes)[^\/?]+)/.test (pathName)
+  const helpVsbl = /\/help\/((?!changes)[^/?]+)/.test (pathName)
   const helpIdentifier = pathName.split ('/')[2] ?? ''
 
   return [
-      { name: '広場', to: '/posts', subMenu: [
-	  { name: '一覧', to: '/posts' },
-	  { name: '検索', to: '/posts/search' },
-	  { name: '追加', to: '/posts/new', visible: editable },
-	  { name: '全体履歴', to: '/posts/changes' },
-	  { name: 'ヘルプ', to: '/wiki/ヘルプ:広場' }] },
-      { name: 'タグ', to: '/tags', subMenu: [
-	  { name: 'マスタ', to: '/tags' },
-	  { name: 'ニコニコ連携', to: '/tags/nico' },
-	  { name: '全体履歴', to: '/tags/changes' },
-	  { name: 'ヘルプ', to: '/wiki/ヘルプ:タグ' },
+      { name: message ('square'), to: '/posts', subMenu: [
+	  { name: message ('list'), to: '/posts' },
+	  { name: message ('search'), to: '/posts/search' },
+	  { name: message ('add'), to: '/posts/new', visible: editable },
+	  { name: message ('allHistory'), to: '/posts/changes' },
+	  { name: message ('help'), to: '/wiki/ヘルプ:広場' }] },
+      { name: message ('tags'), to: '/tags', subMenu: [
+	  { name: message ('master'), to: '/tags' },
+	  { name: message ('nicoLinks'), to: '/tags/nico' },
+	  { name: message ('allHistory'), to: '/tags/changes' },
+	  { name: message ('help'), to: '/wiki/ヘルプ:タグ' },
 	  { component: <Separator/>, visible: tagFlg },
 	  { name: `広場 (${ postCount || 0 })`,
 	    to: `/posts?tags=${ encodeURIComponent (tag?.name ?? '') }`,
 	    visible: tagFlg },
-	  { name: '履歴', to: `/tags/changes?id=${ tag?.id }`,
+	  { name: message ('history'), to: `/tags/changes?id=${ tag?.id }`,
 	    visible: tagFlg && tag?.category !== 'nico' }] },
-      { name: '素材', to: '/materials', visible: true, subMenu: [
-	  { name: '一覧', to: '/materials' },
-	  { name: '追加', to: '/materials/new', visible: editable },
-	  { name: '抑止', to: '/materials/suppressions',
+      { name: message ('materials'), to: '/materials', visible: true, subMenu: [
+	  { name: message ('list'), to: '/materials' },
+	  { name: message ('add'), to: '/materials/new', visible: editable },
+	  { name: message ('suppressions'), to: '/materials/suppressions',
 	    visible: user?.role === 'admin' },
-	  { name: '全体履歴', to: '/materials/changes' },
-	  { name: 'ヘルプ', to: '/wiki/ヘルプ:素材管理' },
+	  { name: message ('allHistory'), to: '/materials/changes' },
+	  { name: message ('help'), to: '/wiki/ヘルプ:素材管理' },
 	  { component: <Separator/>, visible: materialFlg },
 	  { name: `広場 (${ postCount || 0 })`,
 	    to: `/posts?tags=${ encodeURIComponent (material?.tag?.name ?? '') }`,
 	    visible: materialFlg && Boolean (material?.tag) },
-	  { name: '履歴', to: `/materials/changes?material_id=${ material?.id }`,
+	  { name: message ('history'), to: `/materials/changes?material_id=${ material?.id }`,
 	    visible: materialFlg }] },
-      { name: 'Wiki', to: '/wiki', subMenu: [
-	  { name: '検索', to: '/wiki' },
-	  { name: '新規', to: '/wiki/new', visible: editable },
-	  { name: '全体履歴', to: '/wiki/changes' },
-	  { name: 'ヘルプ', to: '/wiki/ヘルプ:Wiki' },
+      { name: message ('wiki'), to: '/wiki', subMenu: [
+	  { name: message ('search'), to: '/wiki' },
+	  { name: message ('create'), to: '/wiki/new', visible: editable },
+	  { name: message ('allHistory'), to: '/wiki/changes' },
+	  { name: message ('help'), to: '/wiki/ヘルプ:Wiki' },
 	  { component: <Separator/>, visible: wikiPageFlg },
 	  { name: `広場 (${ postCount || 0 })`, to: `/posts?tags=${ wikiTitle }`,
 	    visible: wikiPageFlg },
-	  { name: '履歴', to: `/wiki/changes?id=${ wikiId }`, visible: wikiPageFlg },
-	  { name: '編輯', to: `/wiki/${ wikiId || wikiTitle }/edit`,
+	  { name: message ('history'), to: `/wiki/changes?id=${ wikiId }`, visible: wikiPageFlg },
+	  { name: message ('edit'), to: `/wiki/${ wikiId || wikiTitle }/edit`,
 	    visible: wikiPageFlg && editable }] },
-      { name: 'ヘルプ', to: '/help/home', base: '/help', subMenu: [
-	  { name: '検索', to: '/help' },
-	  { name: '全体履歴', to: '/help/changes' },
+      { name: message ('help'), to: '/help/home', base: '/help', subMenu: [
+	  { name: message ('search'), to: '/help' },
+	  { name: message ('allHistory'), to: '/help/changes' },
 	  { component: <Separator/>, visible: helpVsbl },
-	  { name: '履歴',
+	  { name: message ('history'),
 	    to: `/help/changes?identifier=${ helpIdentifier }`,
 	    visible: helpVsbl }] },
-      { name: 'おたのしみ', visible: false, subMenu: [
-	  { name: '上映会 (β)', to: '/theatres/1' },
-	  { name: 'グカネータ (β)', to: '/gekanator' }] },
-      { name: 'ユーザ', to: '/users/settings', visible: false, subMenu: [
-	  { name: '一覧', to: '/users', visible: false },
-	  { name: 'お前', to: `/users/${ user?.id }`, visible: false },
-	  { name: '設定', to: '/users/settings', visible: Boolean (user) }] },
-      { name: '法規', visible: false, subMenu: [
-	  { name: '利用規約', to: '/tos' }] }]
+      { name: message ('entertainment'), visible: false, subMenu: [
+	  { name: message ('theatre'), to: '/theatres/1' },
+	  { name: message ('gekanator'), to: '/gekanator' }] },
+      { name: message ('users'), to: '/users/settings', visible: false, subMenu: [
+	  { name: message ('list'), to: '/users', visible: false },
+	  { name: message ('currentUser'), to: `/users/${ user?.id }`, visible: false },
+	  { name: message ('settings'), to: '/users/settings', visible: Boolean (user) }] },
+      { name: message ('legal'), visible: false, subMenu: [
+	  { name: message ('terms'), to: '/tos' }] }]
 }
 
 
 const TopNav: FC<Props> = ({ user }) => {
+  const locale = useLocale ()
+  const navigate = useLocaleNavigate ()
   const location = useLocation ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
@@ -151,25 +164,26 @@ const TopNav: FC<Props> = ({ user }) => {
 
   const { data: wikiPage } = useQuery ({
       enabled: Boolean (wikiIdStr),
-      queryKey: wikiKeys.show (wikiIdStr, { }),
-      queryFn: () => fetchWikiPage (wikiIdStr, { }) })
+      queryKey: wikiKeys.show (wikiIdStr, { }, locale),
+      queryFn: () => fetchWikiPage (wikiIdStr, { }, locale) })
 
-  const tagFlg = /^\/tags\/\d+/.test (location.pathname)
-  const materialFlg = /^\/materials\/\d+/.test (location.pathname)
+  const pathname = unlocalisedPath (location.pathname)
+  const tagFlg = /^\/tags\/\d+/.test (pathname)
+  const materialFlg = /^\/materials\/\d+/.test (pathname)
   const effectiveTitle = (((tagFlg || materialFlg)
-			   ? location.pathname.split ('/')[2]
+			   ? pathname.split ('/')[2]
 			   : wikiPage?.title)
 			  ?? '')
 
   const { data: tag } = useQuery ({
       enabled: Boolean (effectiveTitle),
-      queryKey: tagsKeys.show (effectiveTitle),
-      queryFn: () => (tagFlg ? fetchTag : fetchTagByName) (effectiveTitle) })
+      queryKey: tagsKeys.show (effectiveTitle, locale),
+      queryFn: () => (tagFlg ? fetchTag : fetchTagByName) (effectiveTitle, locale) })
 
   const { data: material } = useQuery ({
       enabled: Boolean (effectiveTitle),
-      queryKey: materialsKeys.show (effectiveTitle),
-      queryFn: () => fetchMaterial (effectiveTitle) })
+      queryKey: materialsKeys.show (effectiveTitle, locale),
+      queryFn: () => fetchMaterial (effectiveTitle, locale) })
 
   const menu = menuOutline ({ tag, material, wikiId, user, pathName: location.pathname })
   const visibleMenu = menu.filter ((item): item is MenuVisibleItem => item.visible ?? true)
@@ -177,7 +191,7 @@ const TopNav: FC<Props> = ({ user }) => {
       !(item.visible ?? true)
       || item.subMenu.filter (subItem => subItem.visible ?? true).length > 0)
   const activeIdx =
-      visibleMenu.findIndex (item => location.pathname.startsWith (item.base || item.to))
+      visibleMenu.findIndex (item => pathname.startsWith (item.base || item.to))
   const submenuHeight = moreVsbl ? 40 * moreMenu.length : (activeIdx < 0 ? 0 : 40)
   const topNavTransition =
       animationsOff
@@ -298,7 +312,7 @@ const TopNav: FC<Props> = ({ user }) => {
   return (
       <div ref={layoutRef} className="top-nav-layout w-full min-w-0 shrink-0"
 	  data-desktop={desktop} data-density={density}>
-	<nav ref={barRef}
+	<nav ref={barRef} lang={uiMessageLanguage (locale, 'square')}
 	    className="top-nav-root top-nav-bar flex justify-between items-center w-full">
 	  <div className="top-nav-primary flex shrink-0 items-center h-12">
 	    <PrefetchLink
@@ -373,6 +387,22 @@ const TopNav: FC<Props> = ({ user }) => {
 	  </button>
 	</nav>
 
+	<div className="flex min-w-0 justify-end px-2">
+	  <select
+	      aria-label={LOCALE_LABELS[locale]}
+	      value={locale}
+	      className="max-w-full rounded border border-gray-300 bg-white text-xs text-gray-900
+			 dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100"
+	      onChange={event => {
+		const nextLocale = event.target.value
+		if (isLocaleCode (nextLocale))
+		  navigate (switchLocalePath (location, nextLocale))
+	      }}>
+	    {SUPPORTED_LOCALES.map (code => (
+		<option key={code} value={code}>{LOCALE_LABELS[code]}</option>))}
+	  </select>
+	</div>
+
 	<AnimatePresence initial={false}>
 	  <motion.div
 	      key="submenu-shell"

frontend/src/components/WikiBody.tsx

@@ -3,6 +3,7 @@ import { useMemo } from 'react'
 import ReactMarkdown from 'react-markdown'
 import remarkGFM from 'remark-gfm'
 
+import { useLocale } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import { wikiKeys } from '@/lib/queryKeys'
 import remarkWikiAutoLink from '@/lib/remark-wiki-autolink'
@@ -26,10 +27,11 @@ const mdComponents = { a: (({ href, children }) => (
 
 
 const WikiBody: FC<Props> = ({ title, body }) => {
+  const locale = useLocale ()
   const { data } = useQuery ({
       enabled: Boolean (body),
-      queryKey: wikiKeys.index ({ }),
-      queryFn: () => fetchWikiPages ({ }) })
+      queryKey: wikiKeys.index ({ }, locale),
+      queryFn: () => fetchWikiPages ({ }, locale) })
   const pageNames = (data ?? []).map (page => page.title).sort ((a, b) => b.length - a.length)
 
   const remarkPlugins = useMemo (

frontend/src/components/common/TagInput.tsx

@@ -1,5 +1,6 @@
 import { useState } from 'react'
 
+import { useLocale } from '@/lib/useLocale'
 import TagSearchBox from '@/components/TagSearchBox'
 import { apiGet } from '@/lib/api'
 import { inputClass } from '@/lib/utils'
@@ -24,6 +25,7 @@ const TagInput: FC<Props> = ({
     value,
     setValue,
 }) => {
+  const locale = useLocale ()
   const [activeIndex, setActiveIndex] = useState (-1)
   const [suggestions, setSuggestions] = useState<Tag[]> ([])
   const [suggestionsVsbl, setSuggestionsVsbl] = useState (false)
@@ -39,7 +41,7 @@ const TagInput: FC<Props> = ({
 	return
       }
 
-    const data = await apiGet<Tag[]> ('/tags/autocomplete', { params: {
+    const data = await apiGet<Tag[]> ('/tags/autocomplete', { locale, params: {
 	q,
 	nico: includeNico ? '1' : '0' } })
     const nextSuggestions = data.filter (t => t.postCount > 0)

frontend/src/components/dialogues/DialogueProvider.tsx

@@ -88,8 +88,9 @@ const DialogueProvider: FC<Props> = ({ children }) => {
     })
     setPendingIds (current => current.filter (pendingId => pendingId !== id))
     setFormActions (current => {
-	const { [id]: _, ...rest } = current
-	return rest
+      const rest = { ...current }
+      delete rest[id]
+      return rest
     })
     delete formControls.current[id]
   }, [])

frontend/src/lib/api.ts

@@ -5,7 +5,10 @@ import { API_BASE_URL } from '@/config'
 
 import type { AxiosError, AxiosRequestConfig } from 'axios'
 
+import type { LocaleCode } from '@/lib/locales'
+
 type Opt = {
+    locale?:        LocaleCode
     params?:        AxiosRequestConfig['params']
     headers?:       Record<string, string>
     signal?:        AbortSignal
@@ -14,10 +17,14 @@ type Opt = {
 const client = axios.create ({ baseURL: API_BASE_URL })
 
 
-const withUserCode = (opt?: Opt): Opt => ({
-    ...opt,
-    headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '',
-	       ...(opt?.headers ?? { }) } })
+const withUserCode = (opt?: Opt): AxiosRequestConfig => {
+  const { locale, ...config } = opt ?? { }
+  return {
+      ...config,
+      ...(locale != null && { params: { locale, ...config.params } }),
+      headers: { 'X-Transfer-Code': localStorage.getItem ('user_code') ?? '',
+		 ...(config.headers ?? { }) } }
+}
 
 
 const apiP = async <T> (

frontend/src/lib/gekanator.ts

@@ -1,5 +1,6 @@
 import { apiGet, apiPost } from '@/lib/api'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { Post } from '@/types'
 
 export type GekanatorAnswerValue =
@@ -411,24 +412,30 @@ export const storeGekanatorQuestion = (
   exampleAnswers: question.exampleAnswers })
 
 
-export const fetchGekanatorPosts = async (): Promise<Post[]> => {
-  const data = await apiGet<{ posts: Post[] }> ('/gekanator/posts')
+export const fetchGekanatorPosts = async (
+    locale: LocaleCode = 'ja',
+): Promise<Post[]> => {
+  const data = await apiGet<{ posts: Post[] }> ('/gekanator/posts', { locale })
   return data.posts
 }
 
 
-export const fetchGekanatorQuestions = async (): Promise<StoredGekanatorQuestion[]> => {
-  const data = await apiGet<{ questions: StoredGekanatorQuestion[] }> ('/gekanator/questions')
+export const fetchGekanatorQuestions = async (
+    locale: LocaleCode = 'ja',
+): Promise<StoredGekanatorQuestion[]> => {
+  const data = await apiGet<{ questions: StoredGekanatorQuestion[] }> ('/gekanator/questions', { locale })
   return data.questions
 }
 
 
 export const fetchGekanatorExtraQuestions = async (
     gameId:  number,
-    nonce?: string): Promise<GekanatorExtraQuestion[]> => {
+    nonce?: string,
+    locale: LocaleCode = 'ja',
+): Promise<GekanatorExtraQuestion[]> => {
   const data = await apiGet<{ questions: GekanatorExtraQuestion[] }> (
       `/gekanator/games/${ gameId }/extra_questions`,
-      { params: nonce ? { nonce } : undefined })
+      { locale, params: nonce ? { nonce } : undefined })
   return data.questions
 }
 
@@ -606,7 +613,8 @@ export const buildGekanatorQuestions = (
 }
 
 
-export const saveGekanatorGame = async ({
+export const saveGekanatorGame = async (
+    {
     guessedPostId,
     correctPostId,
     answers,
@@ -614,7 +622,9 @@ export const saveGekanatorGame = async ({
     guessedPostId:  number
     correctPostId:  number
     answers:        GekanatorAnswerLog[]
-}): Promise<{ id: number; learnedExampleCount: number }> =>
+},
+    locale: LocaleCode = 'ja',
+): Promise<{ id: number; learnedExampleCount: number }> =>
     await apiPost ('/gekanator/games', {
 	guessed_post_id: guessedPostId,
 	correct_post_id: correctPostId,
@@ -627,10 +637,11 @@ export const saveGekanatorGame = async ({
 	    effective_question: answer.effectiveQuestion,
 	    learning_question: answer.learningQuestion,
 	    answer: answer.answer,
-	    original_answer: answer.originalAnswer })) })
+	    original_answer: answer.originalAnswer })) }, { locale })
 
 
-export const saveGekanatorQuestionSuggestion = async ({
+export const saveGekanatorQuestionSuggestion = async (
+    {
     gekanatorGameId,
     existingQuestionId,
     questionText,
@@ -640,22 +651,27 @@ export const saveGekanatorQuestionSuggestion = async ({
     existingQuestionId?: number
     questionText?:    string
     answer:           GekanatorAnswerValue
-}): Promise<{ id: number; count: number }> =>
+},
+    locale: LocaleCode = 'ja',
+): Promise<{ id: number; count: number }> =>
     await apiPost ('/gekanator/question_suggestions', {
 	gekanator_game_id: gekanatorGameId,
 	existing_question_id: existingQuestionId,
 	question_text: questionText,
-	answer })
+	answer }, { locale })
 
 
-export const saveGekanatorExtraQuestionAnswers = async ({
+export const saveGekanatorExtraQuestionAnswers = async (
+    {
     gameId,
     answers,
 }: {
     gameId:   number
     answers:  { questionId: number; answer: GekanatorAnswerValue }[]
-}) =>
+},
+    locale: LocaleCode = 'ja',
+) =>
     await apiPost (`/gekanator/games/${ gameId }/extra_question_answers`, {
 	answers: answers.map (item => ({
 	    question_id: item.questionId,
-	    answer: item.answer })) })
+	    answer: item.answer })) }, { locale })

frontend/src/lib/help.ts

@@ -13,12 +13,14 @@ export type HelpEditionText = {
 
 export type HelpSearchEntry = HelpEditionText & {
     identifier: string
+    language?:  string
     edition:    string }
 
 export type HelpEditionContent = HelpEditionText & { content: ComponentType }
 
 export type HelpEditionSource = HelpEditionText & { source: string }
 export type HelpChange = Pick<HelpSearchEntry, 'identifier' | 'edition' | 'title'> & {
+    language?: string
     pred: string | null
     change: 'new' | 'update'
     date: string }
@@ -82,6 +84,15 @@ export const getHelpArticle = async (identifier: string): Promise<ArticleMeta |
 }
 
 
+export const resolveHelpLanguage = (
+    article: ArticleMeta | null | undefined,
+    requestedLanguage: string,
+): string =>
+    article?.languages[requestedLanguage] != null
+    ? requestedLanguage
+    : article?.canonicalLanguage ?? 'ja'
+
+
 export const getHelpEditions = async (
     identifier: string,
     language: string,
@@ -144,7 +155,8 @@ export const getHelpSearchEntries = async (
     language: string,
 ): Promise<HelpSearchEntry[]> => {
   const { default: search } = await import ('virtual:help/search')
-  return search[language] ?? []
+  const contentLanguage = search[language] != null ? language : 'ja'
+  return (search[contentLanguage] ?? []).map (entry => ({ ...entry, language: contentLanguage }))
 }
 
 
@@ -163,13 +175,14 @@ export const getHelpEditionContent = async (
 
 export const getHelpChanges = async (language: string): Promise<HelpChange[]> => {
   const { default: changes } = await import ('virtual:help/changes')
-  return changes[language] ?? []
+  const contentLanguage = changes[language] != null ? language : 'ja'
+  return (changes[contentLanguage] ?? []).map (entry => ({ ...entry, language: contentLanguage }))
 }
 
 
 export const getHelpTitleIndex = async (language: string): Promise<HelpTitleIndex> => {
   const { default: titles } = await import ('virtual:help/titles')
-  return titles[language] ?? { }
+  return titles[language] ?? titles.ja ?? { }
 }
 
 

frontend/src/lib/keyboardShortcuts.ts

@@ -1,3 +1,5 @@
+import { unlocalisedPath } from '@/lib/locales'
+
 export type ShortcutScope =
     | 'global'
     | 'posts'
@@ -308,7 +310,8 @@ export const getConflictingShortcutActionIds = (
   )
 }
 
-export const resolveShortcutScope = (pathname: string): ShortcutScope => {
+export const resolveShortcutScope = (fullPathname: string): ShortcutScope => {
+  const pathname = unlocalisedPath (fullPathname)
   if (pathname.startsWith ('/posts/search'))
     return 'postSearch'
   if (pathname.startsWith ('/posts'))

frontend/src/lib/locales.test.ts

@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest'
+
+import { isLocaleCode,
+	 legacyLocalePath,
+	 localeFromPath,
+	 localiseTo,
+	 switchLocalePath,
+	 unlocalisedPath } from '@/lib/locales'
+
+
+describe ('locale URL generation', () => {
+  it.each (['ja', 'ain-Kana']) ('recognises the supported locale %s', locale => {
+    expect (isLocaleCode (locale)).toBe (true)
+    expect (localeFromPath (`/${ locale }/posts`)).toBe (locale)
+  })
+
+  it.each (['en', 'ain', 'JA', '', undefined]) ('rejects %s', locale => {
+    expect (isLocaleCode (locale)).toBe (false)
+  })
+
+  it.each ([
+      '/posts', '/posts/new', '/posts/search', '/posts/42', '/posts/changes',
+      '/tags', '/tags/42', '/tags/42/deerjikists', '/tags/nico', '/nico/tags', '/tags/changes',
+      '/materials', '/materials/new', '/materials/42', '/materials/changes',
+      '/materials/suppressions', '/theatres/42', '/wiki', '/wiki/new', '/wiki/title',
+      '/wiki/42/edit', '/wiki/42/diff', '/wiki/changes', '/help', '/help/changes',
+      '/help/article', '/help/article/diff', '/users/settings', '/settings',
+      '/tos', '/gekanator', '/more']) ('localises and redirects %s', pathname => {
+    const location = { pathname, search: '?page=2&tag=a%23b', hash: '#section' }
+    expect (localiseTo (pathname, 'ain-Kana')).toBe (`/ain-Kana${ pathname }`)
+    expect (legacyLocalePath (location)).toBe (`/ja${ pathname }${ location.search }#section`)
+    expect (unlocalisedPath (`/ain-Kana${ pathname }`)).toBe (pathname)
+  })
+
+  it ('preserves the resource, query and fragment on locale changes', () => {
+    expect (switchLocalePath ({ pathname: '/ja/tags/42', search: '?x=1&x=2', hash: '#history' },
+	'ain-Kana')).toBe ('/ain-Kana/tags/42?x=1&x=2#history')
+  })
+
+  it ('preserves explicit locales and relative, fragment and external targets', () => {
+    for (const target of ['/ja/posts', '/ain-Kana/posts', '?page=2', '#body',
+	'../changes', 'https://example.com/posts', '//example.com/posts'])
+      expect (localiseTo (target, 'ain-Kana')).toBe (target)
+  })
+
+  it ('preserves structured targets', () => {
+    expect (localiseTo ({ pathname: '/tags/42', search: '?q=one', hash: '#two' }, 'ain-Kana'))
+	.toEqual ({ pathname: '/ain-Kana/tags/42', search: '?q=one', hash: '#two' })
+  })
+
+  it.each (['/en/posts', '/ain/posts', '/unknown', '/api/tags', '/assets/image.png']) (
+      'does not redirect invalid or non-page paths: %s', pathname => {
+	expect (legacyLocalePath ({ pathname, search: '', hash: '' })).toBeNull ()
+      })
+})

frontend/src/lib/locales.ts

@@ -0,0 +1,62 @@
+import type { To } from 'react-router-dom'
+
+export const SUPPORTED_LOCALES = ['ja', 'ain-Kana'] as const
+
+export type LocaleCode = typeof SUPPORTED_LOCALES[number]
+
+export const LOCALE_LABELS: Record<LocaleCode, string> = {
+    ja: '日本語', 'ain-Kana': 'ヤウンモシルン アイヌ イタㇰ' }
+
+const LEGACY_ROOTS = [
+    'posts', 'tags', 'nico', 'theatres', 'materials', 'wiki', 'help',
+    'users', 'settings', 'tos', 'gekanator', 'more']
+
+
+export const isLocaleCode = (value: unknown): value is LocaleCode =>
+    SUPPORTED_LOCALES.some (locale => locale === value)
+
+
+export const localeLanguage = (locale: LocaleCode): 'ja' | 'ain' =>
+    locale === 'ja' ? 'ja' : 'ain'
+
+
+export const localeFromPath = (pathname: string): LocaleCode | null => {
+  const segment = pathname.split (/[/?#]/)[1]
+  return isLocaleCode (segment) ? segment : null
+}
+
+
+export const unlocalisedPath = (pathname: string): string => {
+  const locale = localeFromPath (pathname)
+  return locale == null ? pathname : pathname.slice (locale.length + 1) || '/'
+}
+
+
+export const localiseTo = (to: To, locale: LocaleCode): To => {
+  const pathname = typeof to === 'string' ? to : to.pathname
+  if (pathname == null
+      || !(pathname.startsWith ('/'))
+      || pathname.startsWith ('//')
+      || localeFromPath (pathname) != null)
+    return to
+
+  const localised = `/${ locale }${ pathname }`
+  return typeof to === 'string' ? localised : { ...to, pathname: localised }
+}
+
+
+export const switchLocalePath = (
+    location: { pathname: string; search: string; hash: string },
+    locale: LocaleCode,
+): string =>
+    `/${ locale }${ unlocalisedPath (location.pathname) }${ location.search }${ location.hash }`
+
+
+export const legacyLocalePath = (
+    location: { pathname: string; search: string; hash: string },
+): string | null => {
+  const root = location.pathname.split ('/')[1]
+  return (location.pathname === '/' || LEGACY_ROOTS.includes (root))
+      ? `/ja${ location.pathname }${ location.search }${ location.hash }`
+      : null
+}

frontend/src/lib/materials.ts

@@ -1,5 +1,6 @@
 import { apiGet, isApiError, apiPost, apiPut } from '@/lib/api'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { Material,
 	      MaterialIndexResponse,
 	      MaterialVersion,
@@ -36,8 +37,10 @@ export const fetchMaterials = async (
       updatedFrom, updatedTo, sort, direction, page,
       tagId, includeDescendants, groupBy,
       limit }: FetchMaterialsParams,
+    locale: LocaleCode = 'ja',
+
 ): Promise<MaterialIndexResponse> =>
-    await apiGet ('/materials', { params: {
+    await apiGet ('/materials', { locale, params: {
 	...(q && { q }),
 	tag_state:   tagState,
 	media_kind:  mediaKind,
@@ -54,10 +57,13 @@ export const fetchMaterials = async (
 	limit} })
 
 
-export const fetchMaterial = async (id: string): Promise<Material | null> => {
+export const fetchMaterial = async (
+    id: string,
+    locale: LocaleCode = 'ja',
+): Promise<Material | null> => {
   try
   {
-    return await apiGet (`/materials/${ id }`)
+    return await apiGet (`/materials/${ id }`, { locale })
   }
   catch (error)
   {
@@ -75,8 +81,10 @@ export const fetchMaterialChanges = async (
 	eventType?:  string
 	page:        number
 	limit:       number
-    }): Promise<MaterialChangesResponse> =>
-    await apiGet ('/materials/versions', { params: {
+    },
+    locale: LocaleCode = 'ja',
+): Promise<MaterialChangesResponse> =>
+    await apiGet ('/materials/versions', { locale, params: {
 	...(materialId && { material_id: materialId }),
 	...(tag && { tag }),
 	...(eventType && { event_type: eventType }),
@@ -85,19 +93,23 @@ export const fetchMaterialChanges = async (
 
 
 export const fetchMaterialTagTree = async (
-    { parentId, materialFilter }: FetchMaterialTreeParams): Promise<MaterialSidebarTag[]> =>
-    await apiGet ('/tags/with-depth', { params: {
+    { parentId, materialFilter }: FetchMaterialTreeParams,
+    locale: LocaleCode = 'ja',
+): Promise<MaterialSidebarTag[]> =>
+    await apiGet ('/tags/with-depth', { locale, params: {
 	...(parentId != null && { parent: String (parentId) }),
 	material_filter: materialFilter} })
 
 
 export const fetchMaterialTagByName = async (
     name: string,
-    materialFilter: MaterialFilter): Promise<MaterialTagTree | null> => {
+    materialFilter: MaterialFilter,
+    locale: LocaleCode = 'ja',
+): Promise<MaterialTagTree | null> => {
   try
   {
     return await apiGet (`/tags/name/${ encodeURIComponent (name) }/materials`,
-			 { params: { material_filter: materialFilter } })
+			 { locale, params: { material_filter: materialFilter } })
   }
   catch (error)
   {
@@ -108,19 +120,26 @@ export const fetchMaterialTagByName = async (
 }
 
 
-export const createMaterial = async (formData: FormData): Promise<Material> =>
-    await apiPost ('/materials', formData)
+export const createMaterial = async (
+    formData: FormData,
+    locale: LocaleCode = 'ja',
+): Promise<Material> =>
+    await apiPost ('/materials', formData, { locale })
 
 
 export const updateMaterial = async (
     id: string,
-    formData: FormData): Promise<Material> =>
-    await apiPut (`/materials/${ id }`, formData)
+    formData: FormData,
+    locale: LocaleCode = 'ja',
+): Promise<Material> =>
+    await apiPut (`/materials/${ id }`, formData, { locale })
 
 
 export const fetchMaterialSyncSuppressions =
-    async (): Promise<MaterialSyncSuppressionResponse> =>
-        await apiGet ('/materials/suppressions')
+    async (
+    locale: LocaleCode = 'ja',
+): Promise<MaterialSyncSuppressionResponse> =>
+        await apiGet ('/materials/suppressions', { locale })
 
 
 export const createMaterialSyncSuppression = async (
@@ -130,10 +149,12 @@ export const createMaterialSyncSuppression = async (
         drivePath?:   string
         driveFileId?: string
         reason:       string
-    }): Promise<MaterialSyncSuppression> =>
+    },
+    locale: LocaleCode = 'ja',
+): Promise<MaterialSyncSuppression> =>
     await apiPost ('/materials/suppressions', {
         source_kind:   payload.sourceKind,
         source_uri:    payload.sourceUri,
         drive_path:    payload.drivePath,
         drive_file_id: payload.driveFileId,
-        reason:        payload.reason})
+        reason:        payload.reason}, { locale })

frontend/src/lib/messages.ts

@@ -0,0 +1,38 @@
+import type { LocaleCode } from '@/lib/locales'
+
+const japanese = {
+    square: '広場',
+    list: '一覧',
+    search: '検索',
+    add: '追加',
+    history: '履歴',
+    allHistory: '全体履歴',
+    help: 'ヘルプ',
+    tags: 'タグ',
+    master: 'マスタ',
+    nicoLinks: 'ニコニコ連携',
+    materials: '素材',
+    suppressions: '抑止',
+    wiki: 'Wiki',
+    create: '新規',
+    edit: '編輯',
+    entertainment: 'おたのしみ',
+    theatre: '上映会 (β)',
+    gekanator: 'グカネータ (β)',
+    users: 'ユーザ',
+    currentUser: 'お前',
+    settings: '設定',
+    legal: '法規',
+    terms: '利用規約' }
+
+export type UIMessageKey = keyof typeof japanese
+
+const ainu: Partial<Record<UIMessageKey, string>> = { }
+
+
+export const uiMessage = (locale: LocaleCode, key: UIMessageKey): string =>
+    (locale === 'ain-Kana' ? ainu[key] : null) ?? japanese[key]
+
+
+export const uiMessageLanguage = (locale: LocaleCode, key: UIMessageKey): 'ja' | 'ain' =>
+    (locale === 'ain-Kana' && ainu[key] != null) ? 'ain' : 'ja'

frontend/src/lib/posts.ts

@@ -1,14 +1,17 @@
 import { apiDelete, apiGet, apiPost, apiPut } from '@/lib/api'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { FetchPostsParams, Post, PostVersion } from '@/types'
 
 
 export const fetchPosts = async (
     { url, title, tags, match, createdFrom, createdTo, updatedFrom, updatedTo,
-      originalCreatedFrom, originalCreatedTo, page, limit, order }: FetchPostsParams): Promise<{
+      originalCreatedFrom, originalCreatedTo, page, limit, order }: FetchPostsParams,
+    locale: LocaleCode = 'ja',
+): Promise<{
     posts:  Post[]
     count:  number }> =>
-	await apiGet ('/posts', { params: {
+	await apiGet ('/posts', { locale, params: {
 	    ...(url && { url }),
 	    ...(title && { title }),
 	    ...(tags && { tags }),
@@ -24,7 +27,10 @@ export const fetchPosts = async (
 	    ...(order && { order }) } })
 
 
-export const fetchPost = async (id: string): Promise<Post> => await apiGet (`/posts/${ id }`)
+export const fetchPost = async (
+    id: string,
+    locale: LocaleCode = 'ja',
+): Promise<Post> => await apiGet (`/posts/${ id }`, { locale })
 
 
 export const fetchPostChanges = async (
@@ -33,10 +39,12 @@ export const fetchPostChanges = async (
 	tag?:         string
 	externalTag?: string
 	page:         number
-	limit:        number }): Promise<{
+	limit:        number },
+    locale: LocaleCode = 'ja',
+): Promise<{
     versions: PostVersion[]
     count:    number }> =>
-	await apiGet ('/posts/versions', { params: { ...(post && { post }),
+	await apiGet ('/posts/versions', { locale, params: { ...(post && { post }),
 						     ...(tag && { tag }),
 						     ...(externalTag && {
 						       external_tag: externalTag }),
@@ -54,7 +62,9 @@ export const updatePost = async (
     { baseVersionNo, force, merge }: {
 	baseVersionNo?: number
 	force?:         boolean
-	merge?:         boolean }) =>
+	merge?:         boolean },
+    locale: LocaleCode = 'ja',
+) =>
     await apiPut<Post> (
 	`/posts/${ post.id }`,
 	{ title: post.title,
@@ -63,7 +73,7 @@ export const updatePost = async (
 	  parent_post_ids: post.parentPostIds,
 	  original_created_from: post.originalCreatedFrom,
 	  original_created_before: post.originalCreatedBefore },
-	{ params: { ...(baseVersionNo && { base_version_no: String (baseVersionNo) }),
+	{ locale, params: { ...(baseVersionNo && { base_version_no: String (baseVersionNo) }),
 	            force: force ? '1' : '0',
 		    merge: merge ? '1' : '0' } })
 

frontend/src/lib/prefetchers.ts

@@ -5,7 +5,9 @@ import { getHelpArticle,
 	 getHelpChanges,
 	 getHelpDiff,
 	 getHelpEditionContent,
-	 getHelpSearchEntries } from '@/lib/help'
+	 getHelpSearchEntries,
+	 resolveHelpLanguage } from '@/lib/help'
+import { localeFromPath, localeLanguage, unlocalisedPath } from '@/lib/locales'
 import { fetchPost, fetchPosts, fetchPostChanges } from '@/lib/posts'
 import { helpKeys, postsKeys, tagsKeys, wikiKeys } from '@/lib/queryKeys'
 import { fetchNicoTags, fetchTagByName, fetchTag, fetchTagChanges, fetchTags } from '@/lib/tags'
@@ -13,9 +15,10 @@ import { fetchWikiPage,
 	 fetchWikiPageByTitle,
 	 fetchWikiPages } from '@/lib/wiki'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { Category, FetchPostsOrder, FetchTagsOrder } from '@/types'
 
-type Prefetcher = (qc: QueryClient, url: URL) => Promise<void>
+type Prefetcher = (qc: QueryClient, url: URL, locale: LocaleCode) => Promise<void>
 
 const mPost = match<{ id: string }> ('/posts/:id')
 const mWiki = match<{ title: string }> ('/wiki/:title')
@@ -28,25 +31,25 @@ const boolFromQuery = (value: string | null): boolean =>
     ['1', 'true', 'on', 'yes', ''].includes ((value ?? '').toLowerCase ())
 
 
-const prefetchHelpIndex: Prefetcher = async qc => {
+const prefetchHelpIndex: Prefetcher = async (qc, url, locale) => {
   await qc.prefetchQuery ({
-      queryKey: helpKeys.index ('ja'),
-      queryFn: () => getHelpSearchEntries ('ja'),
+      queryKey: helpKeys.index (localeLanguage (locale)),
+      queryFn: () => getHelpSearchEntries (localeLanguage (locale)),
       staleTime: Infinity })
 }
 
 
-const prefetchHelpShow: Prefetcher = async (qc, url) => {
+const prefetchHelpShow: Prefetcher = async (qc, url, locale) => {
   const matched = mHelp (url.pathname)
   if (!(matched))
     return
 
   const { identifier } = matched.params
-  const language = 'ja'
   const article = await qc.fetchQuery ({
       queryKey: helpKeys.article (identifier),
       queryFn: () => getHelpArticle (identifier),
       staleTime: Infinity })
+  const language = resolveHelpLanguage (article, localeLanguage (locale))
   const edition = url.searchParams.get ('edition') ?? article?.languages[language]?.currentEdition
   if (edition == null)
     return
@@ -58,15 +61,15 @@ const prefetchHelpShow: Prefetcher = async (qc, url) => {
 }
 
 
-const prefetchHelpChanges: Prefetcher = async qc => {
+const prefetchHelpChanges: Prefetcher = async (qc, url, locale) => {
   await qc.prefetchQuery ({
-      queryKey: helpKeys.changes ('ja'),
-      queryFn: () => getHelpChanges ('ja'),
+      queryKey: helpKeys.changes (localeLanguage (locale)),
+      queryFn: () => getHelpChanges (localeLanguage (locale)),
       staleTime: Infinity })
 }
 
 
-const prefetchHelpDiff: Prefetcher = async (qc, url) => {
+const prefetchHelpDiff: Prefetcher = async (qc, url, locale) => {
   const matched = mHelpDiff (url.pathname)
   const fromValues = url.searchParams.getAll ('from')
   const toValues = url.searchParams.getAll ('to')
@@ -76,23 +79,28 @@ const prefetchHelpDiff: Prefetcher = async (qc, url) => {
     return
 
   const { identifier } = matched.params
+  const article = await qc.fetchQuery ({
+      queryKey: helpKeys.article (identifier),
+      queryFn: () => getHelpArticle (identifier),
+      staleTime: Infinity })
+  const language = resolveHelpLanguage (article, localeLanguage (locale))
   await qc.prefetchQuery ({
-      queryKey: helpKeys.diff (identifier, 'ja', from, to),
-      queryFn: () => getHelpDiff (identifier, 'ja', from, to),
+      queryKey: helpKeys.diff (identifier, language, from, to),
+      queryFn: () => getHelpDiff (identifier, language, from, to),
       staleTime: Infinity })
 }
 
 
-const prefetchWikiPagesIndex: Prefetcher = async (qc, url) => {
+const prefetchWikiPagesIndex: Prefetcher = async (qc, url, locale) => {
   const title = url.searchParams.get ('title') ?? ''
 
   await qc.prefetchQuery ({
-      queryKey: wikiKeys.index ({ title }),
-      queryFn:  () => fetchWikiPages ({ title }) })
+      queryKey: wikiKeys.index ({ title }, locale),
+      queryFn:  () => fetchWikiPages ({ title }, locale) })
 }
 
 
-const prefetchWikiPageShow: Prefetcher = async (qc, url) => {
+const prefetchWikiPageShow: Prefetcher = async (qc, url, locale) => {
   const m = mWiki (url.pathname)
   if (!(m))
     return
@@ -101,29 +109,29 @@ const prefetchWikiPageShow: Prefetcher = async (qc, url) => {
   const version = url.searchParams.get ('version') || undefined
 
   const wikiPage = await qc.fetchQuery ({
-      queryKey: wikiKeys.show (title, { version }),
-      queryFn:  () => fetchWikiPageByTitle (title, { version }) })
+      queryKey: wikiKeys.show (title, { version }, locale),
+      queryFn:  () => fetchWikiPageByTitle (title, { version }, locale) })
 
   if (wikiPage)
     {
       const effectiveId = String (wikiPage.id ?? '')
       await qc.prefetchQuery ({
-	  queryKey: wikiKeys.show (effectiveId, { }),
-	  queryFn: () => fetchWikiPage (effectiveId, { } ) })
+	  queryKey: wikiKeys.show (effectiveId, { }, locale),
+	  queryFn: () => fetchWikiPage (effectiveId, { }, locale ) })
 
       if (wikiPage.body)
 	{
 	  await qc.prefetchQuery ({
-	      queryKey: wikiKeys.index ({ }),
-	      queryFn: () => fetchWikiPages ({ }) })
+	      queryKey: wikiKeys.index ({ }, locale),
+	      queryFn: () => fetchWikiPages ({ }, locale) })
 	}
     }
 
   const effectiveTitle = wikiPage?.title ?? title
 
   await qc.prefetchQuery ({
-      queryKey: tagsKeys.show (effectiveTitle),
-      queryFn: () => fetchTagByName (effectiveTitle) })
+      queryKey: tagsKeys.show (effectiveTitle, locale),
+      queryFn: () => fetchTagByName (effectiveTitle, locale) })
 
   if (version)
     return
@@ -143,12 +151,12 @@ const prefetchWikiPageShow: Prefetcher = async (qc, url) => {
       updatedTo: '',
       order: 'original_created_at:desc' } as const
   await qc.prefetchQuery ({
-      queryKey: postsKeys.index (p),
-      queryFn:  () => fetchPosts (p) })
+      queryKey: postsKeys.index (p, locale),
+      queryFn:  () => fetchPosts (p, locale) })
 }
 
 
-const prefetchPostsIndex: Prefetcher = async (qc, url) => {
+const prefetchPostsIndex: Prefetcher = async (qc, url, locale) => {
   const tags = url.searchParams.get ('tags') ?? ''
   const qURL = url.searchParams.get ('url') ?? ''
   const title = url.searchParams.get ('title') ?? ''
@@ -169,12 +177,12 @@ const prefetchPostsIndex: Prefetcher = async (qc, url) => {
       updatedFrom, updatedTo, order }
 
   await qc.prefetchQuery ({
-      queryKey: postsKeys.index (keys),
-      queryFn:  () => fetchPosts (keys) })
+      queryKey: postsKeys.index (keys, locale),
+      queryFn:  () => fetchPosts (keys, locale) })
 }
 
 
-const prefetchPostShow: Prefetcher = async (qc, url) => {
+const prefetchPostShow: Prefetcher = async (qc, url, locale) => {
   const m = mPost (url.pathname)
   if (!(m))
     return
@@ -182,12 +190,12 @@ const prefetchPostShow: Prefetcher = async (qc, url) => {
   const { id } = m.params
 
   await qc.prefetchQuery ({
-      queryKey: postsKeys.show (id),
-      queryFn:  () => fetchPost (id) })
+      queryKey: postsKeys.show (id, locale),
+      queryFn:  () => fetchPost (id, locale) })
 }
 
 
-const prefetchPostChanges: Prefetcher = async (qc, url) => {
+const prefetchPostChanges: Prefetcher = async (qc, url, locale) => {
   const id = url.searchParams.get ('id')
   const tag = url.searchParams.get ('tag')
   const externalTag = url.searchParams.get ('external_tag')
@@ -197,23 +205,23 @@ const prefetchPostChanges: Prefetcher = async (qc, url) => {
   if (tag)
     {
       await qc.prefetchQuery ({
-	  queryKey: tagsKeys.show (tag),
-	  queryFn: () => fetchTag (tag) })
+	  queryKey: tagsKeys.show (tag, locale),
+	  queryFn: () => fetchTag (tag, locale) })
     }
 
   await qc.prefetchQuery ({
       queryKey: postsKeys.changes ({ ...(id && { id }),
 				     ...(tag && { tag }),
 				     ...(externalTag && { externalTag }),
-				     page, limit }),
+				     page, limit }, locale),
       queryFn:  () => fetchPostChanges ({ ...(id && { id }),
 					  ...(tag && { tag }),
 					  ...(externalTag && { externalTag }),
-					  page, limit }) })
+					  page, limit }, locale) })
 }
 
 
-const prefetchTagsIndex: Prefetcher = async (qc, url) => {
+const prefetchTagsIndex: Prefetcher = async (qc, url, locale) => {
   const postRaw = url.searchParams.get ('post')
   const post = postRaw ? Number (postRaw) : null
   const name = url.searchParams.get ('name') ?? ''
@@ -237,12 +245,12 @@ const prefetchTagsIndex: Prefetcher = async (qc, url) => {
       updatedFrom, updatedTo, deprecated, page, limit, order }
 
   await qc.prefetchQuery ({
-      queryKey: tagsKeys.index (keys),
-      queryFn:  () => fetchTags (keys) })
+      queryKey: tagsKeys.index (keys, locale),
+      queryFn:  () => fetchTags (keys, locale) })
 }
 
 
-const prefetchNicoTagsIndex: Prefetcher = async (qc, url) => {
+const prefetchNicoTagsIndex: Prefetcher = async (qc, url, locale) => {
   const keys = {
       name:       url.searchParams.get ('name') ?? '',
       linkedTag:  url.searchParams.get ('linked_tag') ?? '',
@@ -255,12 +263,12 @@ const prefetchNicoTagsIndex: Prefetcher = async (qc, url) => {
 		  | 'updated_at:asc' | 'updated_at:desc' }
 
   await qc.prefetchQuery ({
-      queryKey: tagsKeys.nicoIndex (keys),
-      queryFn:  () => fetchNicoTags (keys) })
+      queryKey: tagsKeys.nicoIndex (keys, locale),
+      queryFn:  () => fetchNicoTags (keys, locale) })
 }
 
 
-const prefetchTagShow: Prefetcher = async (qc, url) => {
+const prefetchTagShow: Prefetcher = async (qc, url, locale) => {
   const m = mTag (url.pathname)
   if (!(m))
     return
@@ -268,19 +276,19 @@ const prefetchTagShow: Prefetcher = async (qc, url) => {
   const { id } = m.params
 
   await qc.prefetchQuery ({
-      queryKey: tagsKeys.show (id),
-      queryFn:  () => fetchTag (id) })
+      queryKey: tagsKeys.show (id, locale),
+      queryFn:  () => fetchTag (id, locale) })
 }
 
 
-const prefetchTagChanges: Prefetcher = async (qc, url) => {
+const prefetchTagChanges: Prefetcher = async (qc, url, locale) => {
   const id = url.searchParams.get ('id')
   const page = Number (url.searchParams.get ('page') || 1)
   const limit = Number (url.searchParams.get ('limit') || 20)
 
   await qc.prefetchQuery ({
-      queryKey: tagsKeys.changes ({ ...(id && { id }), page, limit }),
-      queryFn: () => fetchTagChanges ({ ...(id && { id }), page, limit }) })
+      queryKey: tagsKeys.changes ({ ...(id && { id }), page, limit }, locale),
+      queryFn: () => fetchTagChanges ({ ...(id && { id }), page, limit }, locale) })
 }
 
 
@@ -313,9 +321,14 @@ export const routePrefetchers: { test: (u: URL) => boolean; run: Prefetcher }[]
 
 export const prefetchForURL = async (qc: QueryClient, urlLike: string): Promise<void> => {
   const u = new URL (urlLike, location.origin)
+  if (u.origin !== location.origin)
+    return
+
+  const locale = localeFromPath (u.pathname) ?? 'ja'
+  u.pathname = unlocalisedPath (u.pathname)
   const r = routePrefetchers.find (x => x.test (u))
   if (!(r))
     return
 
-  await r.run (qc, u)
+  await r.run (qc, u, locale)
 }

frontend/src/lib/queryKeys.ts

@@ -1,3 +1,4 @@
+import type { LocaleCode } from '@/lib/locales'
 import type {
     FetchNicoTagsParams,
     FetchMaterialsParams,
@@ -8,59 +9,125 @@ import type {
 
 export const postsKeys = {
     root:     ['posts'] as const,
-    index:    (p: FetchPostsParams) => ['posts', 'index', p] as const,
-    show:     (id: string) => ['posts', id] as const,
-    related:  (id: string) => ['related', id] as const,
-    changes:  (p: { post?: string
+    index:    (
+    p: FetchPostsParams,
+    locale: LocaleCode = 'ja',
+) => ['posts', locale, 'index', p] as const,
+    show:     (
+    id: string,
+    locale: LocaleCode = 'ja',
+) => ['posts', locale, id] as const,
+    related:  (
+    id: string,
+    locale: LocaleCode = 'ja',
+) => ['related', locale, id] as const,
+    changes:  (
+    p: { post?: string
 		    tag?: string
 		    externalTag?: string
 		    page: number
-		    limit: number }) =>
-		  ['posts', 'changes', p] as const }
+		    limit: number },
+    locale: LocaleCode = 'ja',
+) =>
+		  ['posts', locale, 'changes', p] as const }
 
 export const gekanatorKeys = {
     root:      ['gekanator'] as const,
-    posts:     () => ['gekanator', 'posts'] as const,
-    questions: () => ['gekanator', 'questions'] as const,
-    extraQuestions: (gameId: number, nonce: string) =>
-		       ['gekanator', 'games', gameId, 'extra-questions', nonce] as const }
+    posts:     (
+    locale: LocaleCode = 'ja',
+) => ['gekanator', locale, 'posts'] as const,
+    questions: (
+    locale: LocaleCode = 'ja',
+) => ['gekanator', locale, 'questions'] as const,
+    extraQuestions: (
+    gameId: number,
+    nonce: string,
+    locale: LocaleCode = 'ja',
+) =>
+		       ['gekanator', locale, 'games', gameId, 'extra-questions', nonce] as const }
 
 export const tagsKeys = {
     root:         ['tags'] as const,
-    index:        (p: FetchTagsParams) => ['tags', 'index', p] as const,
+    index:        (
+    p: FetchTagsParams,
+    locale: LocaleCode = 'ja',
+) => ['tags', locale, 'index', p] as const,
     nicoRoot:     ['tags', 'nico'] as const,
-    nicoIndex:    (p: FetchNicoTagsParams) => ['tags', 'nico', 'index', p] as const,
-    externalShow: (id: string) => ['tags', 'nico', id] as const,
-    show:         (name: string) => ['tags', name] as const,
-    changes:      (p: { id?: string; page: number; limit: number }) =>
-		      ['tags', 'changes', p] as const,
-    deerjikists:  (id: string) => ['tags', 'deerjikists', id] as const }
+    nicoIndex:    (
+    p: FetchNicoTagsParams,
+    locale: LocaleCode = 'ja',
+) => ['tags', locale, 'nico', 'index', p] as const,
+    externalShow: (
+    id: string,
+    locale: LocaleCode = 'ja',
+) => ['tags', locale, 'nico', id] as const,
+    show:         (
+    name: string,
+    locale: LocaleCode = 'ja',
+) => ['tags', locale, name] as const,
+    changes:      (
+    p: { id?: string; page: number; limit: number },
+    locale: LocaleCode = 'ja',
+) =>
+		      ['tags', locale, 'changes', p] as const,
+    deerjikists:  (
+    id: string,
+    locale: LocaleCode = 'ja',
+) => ['tags', locale, 'deerjikists', id] as const }
 
 export const materialsKeys = {
     root:         ['materials'] as const,
-    index:        (p: FetchMaterialsParams) => ['materials', 'index', p] as const,
-    changes:      (p: {
+    index:        (
+    p: FetchMaterialsParams,
+    locale: LocaleCode = 'ja',
+) => ['materials', locale, 'index', p] as const,
+    changes:      (
+    p: {
 	materialId?: string
 	tag?:        string
 	eventType?:  string
 	page:        number
 	limit:       number
-    }) => ['materials', 'changes', p] as const,
-    byTagName:     (name: string, materialFilter: MaterialFilter) =>
-		      ['materials', 'tag', name, materialFilter] as const,
-    show:         (id: string) => ['materials', id] as const,
-    suppressions: () => ['materials', 'suppressions'] as const,
-    tree:         (p: {
+    },
+    locale: LocaleCode = 'ja',
+) => ['materials', locale, 'changes', p] as const,
+    byTagName:     (
+    name: string,
+    materialFilter: MaterialFilter,
+    locale: LocaleCode = 'ja',
+) =>
+		      ['materials', locale, 'tag', name, materialFilter] as const,
+    show:         (
+    id: string,
+    locale: LocaleCode = 'ja',
+) => ['materials', locale, id] as const,
+    suppressions: (
+    locale: LocaleCode = 'ja',
+) => ['materials', locale, 'suppressions'] as const,
+    tree:         (
+    p: {
 	parentId?:       number | null
 	materialFilter:  MaterialFilter
-    }) => ['materials', 'tree', p] as const,
-    unclassified: (p: { page?: number; limit?: number } = { }) =>
-		      ['materials', 'unclassified', p] as const}
+    },
+    locale: LocaleCode = 'ja',
+) => ['materials', locale, 'tree', p] as const,
+    unclassified: (
+    p: { page?: number; limit?: number } = { },
+    locale: LocaleCode = 'ja',
+) =>
+		      ['materials', locale, 'unclassified', p] as const}
 
 export const wikiKeys = {
     root:   ['wiki'] as const,
-    index:  (p: { title?: string }) => ['wiki', 'index', p] as const,
-    show:   (title: string, p: { version?: string }) => ['wiki', title, p] as const }
+    index:  (
+    p: { title?: string },
+    locale: LocaleCode = 'ja',
+) => ['wiki', locale, 'index', p] as const,
+    show:   (
+    title: string,
+    p: { version?: string },
+    locale: LocaleCode = 'ja',
+) => ['wiki', locale, title, p] as const }
 
 export const helpKeys = {
     root:    ['help'] as const,

frontend/src/lib/tags.ts

@@ -2,6 +2,7 @@ import { isAxiosError } from 'axios'
 
 import { apiGet } from '@/lib/api'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { Deerjikist,
 	      FetchNicoTagsParams,
 	      FetchTagsParams,
@@ -13,9 +14,11 @@ import type { Deerjikist,
 export const fetchTags = async (
     { post, name, category, postCountGTE, postCountLTE, createdFrom, createdTo,
       updatedFrom, updatedTo, deprecated,
-      page, limit, order }: FetchTagsParams): Promise<{ tags:  Tag[]
+      page, limit, order }: FetchTagsParams,
+    locale: LocaleCode = 'ja',
+): Promise<{ tags:  Tag[]
 	     count: number }> =>
-    await apiGet ('/tags', { params: {
+    await apiGet ('/tags', { locale, params: {
 	...(post != null && { post }),
 	...(name && { name }),
 	...(category && { category }),
@@ -32,9 +35,11 @@ export const fetchTags = async (
 
 
 export const fetchNicoTags = async (
-    { name, linkedTag, linkStatus, page, limit, order }: FetchNicoTagsParams): Promise<{ tags:  NicoTag[]
+    { name, linkedTag, linkStatus, page, limit, order }: FetchNicoTagsParams,
+    locale: LocaleCode = 'ja',
+): Promise<{ tags:  NicoTag[]
 	     count: number }> =>
-    await apiGet ('/tags/nico', { params: {
+    await apiGet ('/tags/nico', { locale, params: {
 	page,
 	limit,
 	name,
@@ -43,10 +48,13 @@ export const fetchNicoTags = async (
 	order } })
 
 
-export const fetchTag = async (id: string): Promise<Tag | null> => {
+export const fetchTag = async (
+    id: string,
+    locale: LocaleCode = 'ja',
+): Promise<Tag | null> => {
   try
   {
-    return await apiGet (`/tags/${ id }`)
+    return await apiGet (`/tags/${ id }`, { locale })
   }
   catch
   {
@@ -55,10 +63,13 @@ export const fetchTag = async (id: string): Promise<Tag | null> => {
 }
 
 
-export const fetchExternalTag = async (id: string): Promise<Tag | null> => {
+export const fetchExternalTag = async (
+    id: string,
+    locale: LocaleCode = 'ja',
+): Promise<Tag | null> => {
   try
   {
-    return await apiGet (`/tags/nico/${ id }`)
+    return await apiGet (`/tags/nico/${ id }`, { locale })
   }
   catch
   {
@@ -67,10 +78,13 @@ export const fetchExternalTag = async (id: string): Promise<Tag | null> => {
 }
 
 
-export const fetchTagByName = async (name: string): Promise<Tag | null> => {
+export const fetchTagByName = async (
+    name: string,
+    locale: LocaleCode = 'ja',
+): Promise<Tag | null> => {
   try
   {
-    return await apiGet (`/tags/name/${ encodeURIComponent (name) }`)
+    return await apiGet (`/tags/name/${ encodeURIComponent (name) }`, { locale })
   }
   catch (error)
   {
@@ -84,12 +98,16 @@ export const fetchTagChanges = async (
     { id, page, limit }: {
 	id?:    string
 	page:   number
-	limit:  number }): Promise<{
+	limit:  number },
+    locale: LocaleCode = 'ja',
+): Promise<{
     versions: TagVersion[]
     count:    number }> =>
-	await apiGet ('/tags/versions', { params: { ...(id && { id }), page, limit } })
+	await apiGet ('/tags/versions', { locale, params: { ...(id && { id }), page, limit } })
 
 
 export const fetchDeerjikistsByTag = async (
-    id: string): Promise<{ tag: Tag; deerjikists: Deerjikist[] }> =>
-    await apiGet (`/tags/${ id }/deerjikists`)
+    id: string,
+    locale: LocaleCode = 'ja',
+): Promise<{ tag: Tag; deerjikists: Deerjikist[] }> =>
+    await apiGet (`/tags/${ id }/deerjikists`, { locale })

frontend/src/lib/useKeyboardShortcuts.tsx

@@ -7,8 +7,9 @@ import {
     useRef,
     useState,
 } from 'react'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocaleNavigate } from '@/lib/useLocale'
 import ShortcutHelpDialog from '@/components/users/ShortcutHelpDialog'
 import {
     DEFAULT_KEY_BINDINGS,
@@ -95,7 +96,7 @@ const focusSearchTarget = (): void => {
 
 export const KeyboardShortcutsProvider = ({ children }: PropsWithChildren) => {
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const [keyboardSettings, setKeyboardSettingsState] =
       useState<ClientKeyboardSettings> (() => getClientKeyboardSettings ())

frontend/src/lib/useLocale.ts

@@ -0,0 +1,27 @@
+import { useCallback } from 'react'
+import { useLocation, useNavigate, useParams } from 'react-router-dom'
+
+import { isLocaleCode, localeFromPath, localiseTo } from '@/lib/locales'
+
+import type { NavigateFunction, NavigateOptions, To } from 'react-router-dom'
+
+import type { LocaleCode } from '@/lib/locales'
+
+
+export const useLocale = (): LocaleCode => {
+  const { locale } = useParams ()
+  const location = useLocation ()
+  return isLocaleCode (locale) ? locale : localeFromPath (location.pathname) ?? 'ja'
+}
+
+
+export const useLocaleNavigate = (): NavigateFunction => {
+  const navigate = useNavigate ()
+  const locale = useLocale ()
+  return useCallback ((to: To | number, options?: NavigateOptions) => {
+    if (typeof to === 'number')
+      navigate (to)
+    else
+      navigate (localiseTo (to, locale), options)
+  }, [locale, navigate])
+}

frontend/src/lib/wiki.ts

@@ -2,26 +2,33 @@ import { isAxiosError } from 'axios'
 
 import { apiGet } from '@/lib/api'
 
+import type { LocaleCode } from '@/lib/locales'
 import type { WikiPage } from '@/types'
 
 
 export const fetchWikiPages = async (
-    { title }:  { title?: string }): Promise<WikiPage[]> =>
-    await apiGet ('/wiki', { params: { title } })
+    { title }:  { title?: string },
+    locale: LocaleCode = 'ja',
+): Promise<WikiPage[]> =>
+    await apiGet ('/wiki', { locale, params: { title } })
 
 
 export const fetchWikiPage = async (
     id:           string,
-    { version }:  { version?: string }): Promise<WikiPage> =>
-    await apiGet (`/wiki/${ id }`, { params: version ? { version } : { } })
+    { version }:  { version?: string },
+    locale: LocaleCode = 'ja',
+): Promise<WikiPage> =>
+    await apiGet (`/wiki/${ id }`, { locale, params: version ? { version } : { } })
 
 
 export const fetchWikiPageByTitle = async (
     title:        string,
-    { version }:  { version?: string }): Promise<WikiPage | null> => {
+    { version }:  { version?: string },
+    locale: LocaleCode = 'ja',
+): Promise<WikiPage | null> => {
   try
   {
-    return await apiGet (`/wiki/title/${ encodeURIComponent (title) }`, { params: { version } })
+    return await apiGet (`/wiki/title/${ encodeURIComponent (title) }`, { locale, params: { version } })
   }
   catch (error)
   {

frontend/src/pages/GekanatorPage.tsx

@@ -3,6 +3,7 @@ import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 
+import { useLocale } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
@@ -3678,6 +3679,7 @@ const expectedAnswerFor = (
 
 
 const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const storedGame = useMemo (loadStoredGame, [])
@@ -3771,8 +3773,8 @@ const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
   const [history, setHistory] = useState<GameSnapshot[]> ([])
 
   const { data: posts = [], isLoading, error } = useQuery ({
-      queryKey: gekanatorKeys.posts (),
-      queryFn: fetchGekanatorPosts,
+      queryKey: gekanatorKeys.posts (locale),
+      queryFn: () => fetchGekanatorPosts (locale),
       refetchOnWindowFocus: false })
   const {
       data: acceptedQuestions = [],
@@ -3780,8 +3782,8 @@ const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
       isLoading: acceptedQuestionsLoading,
       error: acceptedQuestionsError
   } = useQuery ({
-      queryKey: gekanatorKeys.questions (),
-      queryFn: fetchGekanatorQuestions,
+      queryKey: gekanatorKeys.questions (locale),
+      queryFn: () => fetchGekanatorQuestions (locale),
       select: questions => questions.map (restoreGekanatorQuestion),
       refetchOnWindowFocus: false })
   const materialIndex = useMemo (() => buildMaterialIndex (posts), [posts])
@@ -4103,7 +4105,7 @@ const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
   const mascotAsset = mascotAssetByState[mascot]
   const mascotAlt = mascotAltByState[mascot]
   const saveMutation = useMutation ({
-      mutationFn: saveGekanatorGame,
+      mutationFn: (variables: Parameters<typeof saveGekanatorGame>[0]) => saveGekanatorGame (variables, locale),
       onSuccess: (data, variables) => {
 	setRecentGames (storeRecentGameSummary ({
 	    correctPostId: variables.correctPostId,
@@ -4114,9 +4116,9 @@ const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
 	setLearnedExampleCount (data.learnedExampleCount)
 	setResultWon (variables.guessedPostId === variables.correctPostId)}})
   const questionSuggestionMutation = useMutation ({
-      mutationFn: saveGekanatorQuestionSuggestion,
+      mutationFn: (variables: Parameters<typeof saveGekanatorQuestionSuggestion>[0]) => saveGekanatorQuestionSuggestion (variables, locale),
       onSuccess: async data => {
-	await queryClient.refetchQueries ({ queryKey: gekanatorKeys.questions () })
+	await queryClient.refetchQueries ({ queryKey: gekanatorKeys.questions (locale) })
 	setQuestionSuggestionCount (data.count)
 	setQuestionSuggestionEntryMode ('search')
 	setQuestionSuggestionSearch ('')
@@ -4124,9 +4126,9 @@ const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
 	setQuestionSuggestion ('')
 	setQuestionSuggestionAnswer ('yes')}})
   const extraQuestionAnswersMutation = useMutation ({
-      mutationFn: saveGekanatorExtraQuestionAnswers,
+      mutationFn: (variables: Parameters<typeof saveGekanatorExtraQuestionAnswers>[0]) => saveGekanatorExtraQuestionAnswers (variables, locale),
       onSuccess: async () => {
-	await queryClient.refetchQueries ({ queryKey: gekanatorKeys.questions () })
+	await queryClient.refetchQueries ({ queryKey: gekanatorKeys.questions (locale) })
 	setExtraQuestionState ('saved')
 	setPhase ('end')}})
 
@@ -4784,8 +4786,8 @@ const GekanatorPage: FC<{ user: User | null }> = ({ user }) => {
     try
     {
       const questions = await queryClient.fetchQuery ({
-	  queryKey: gekanatorKeys.extraQuestions (gameId, nonce),
-	  queryFn: () => fetchGekanatorExtraQuestions (gameId, nonce) })
+	  queryKey: gekanatorKeys.extraQuestions (gameId, nonce, locale),
+	  queryFn: () => fetchGekanatorExtraQuestions (gameId, nonce, locale) })
       setExtraQuestions (questions.slice (0, 2))
       setExtraQuestionState (questions.length > 0 ? 'ready' : 'empty')
     }

frontend/src/pages/deerjikists/DeerjikistDetailPage.tsx

@@ -2,6 +2,7 @@ import { useQuery, useQueryClient } from '@tanstack/react-query'
 import { useEffect, useMemo, useState } from 'react'
 import { useParams } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import TagLink from '@/components/TagLink'
 import FieldError from '@/components/common/FieldError'
 import FormField from '@/components/common/FormField'
@@ -24,12 +25,13 @@ type DeerjikistFormField =
 
 
 const DeerjikistDetailPage: FC = () => {
+  const locale = useLocale ()
   const { id } = useParams ()
   const tagId = String (id ?? '')
-  const tagKey = tagsKeys.deerjikists (tagId)
+  const tagKey = tagsKeys.deerjikists (tagId, locale)
 
   const { data: qData, isLoading: loading } =
-      useQuery ({ queryKey: tagKey, queryFn: () => fetchDeerjikistsByTag (tagId) })
+      useQuery ({ queryKey: tagKey, queryFn: () => fetchDeerjikistsByTag (tagId, locale) })
   const tag = qData?.tag
   const deerjikists = useMemo (() => qData?.deerjikists ?? [], [qData])
 
@@ -49,7 +51,7 @@ const DeerjikistDetailPage: FC = () => {
     {
       setDisabled (true)
 
-      setData (await apiPut<Deerjikist[]> (`/tags/${ id }/deerjikists`, data))
+      setData (await apiPut<Deerjikist[]> (`/tags/${ id }/deerjikists`, data, { locale }))
       qc.invalidateQueries ({ queryKey: tagsKeys.root })
 
       toast ({ description: '更新しました．' })

frontend/src/pages/help/HelpChangesPage.tsx

@@ -9,6 +9,8 @@ import Pagination from '@/components/common/Pagination'
 import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
 import { getHelpChanges, helpEditionDate } from '@/lib/help'
+import { localeLanguage } from '@/lib/locales'
+import { useLocale } from '@/lib/useLocale'
 import { helpKeys } from '@/lib/queryKeys'
 import { LIST_LIMIT_OPTIONS } from '@/lib/settings'
 import { dateString } from '@/lib/utils'
@@ -17,6 +19,7 @@ import type { FC } from 'react'
 
 
 const HelpChangesPage: FC = () => {
+  const language = localeLanguage (useLocale ())
   const [searchParams, setSearchParams] = useSearchParams ()
   const identifier = searchParams.get ('identifier') ?? ''
   const pageValue = searchParams.get ('page') ?? '1'
@@ -27,8 +30,8 @@ const HelpChangesPage: FC = () => {
   const limit = LIST_LIMIT_OPTIONS.find (
       value => String (value) === searchParams.get ('limit')) ?? 20
   const { data: changes = [], isLoading, error } = useQuery ({
-      queryKey: helpKeys.changes ('ja'),
-      queryFn: () => getHelpChanges ('ja'),
+      queryKey: helpKeys.changes (language),
+      queryFn: () => getHelpChanges (language),
       staleTime: Infinity })
 
   const results = useMemo (() => identifier

frontend/src/pages/help/HelpDetailPage.tsx

@@ -8,7 +8,10 @@ import { SITE_TITLE } from '@/config'
 import { getHelpArticle,
 	 getHelpEditionContent,
 	 helpEditionDate,
-	 parseHelpEdition } from '@/lib/help'
+	 parseHelpEdition,
+	 resolveHelpLanguage } from '@/lib/help'
+import { localeLanguage } from '@/lib/locales'
+import { useLocale } from '@/lib/useLocale'
 import { dateString } from '@/lib/utils'
 import { helpKeys } from '@/lib/queryKeys'
 import NotFound from '@/pages/NotFound'
@@ -17,17 +20,17 @@ import type { FC } from 'react'
 
 
 const HelpDetailPage: FC = () => {
+  const locale = useLocale ()
   const { identifier = '' } = useParams ()
   const location = useLocation ()
 
-  // Locale 対応が入るまでは ja 固定
-  const language = 'ja'
-
   const { data: article, isLoading: articleLoading } = useQuery ({
       queryKey: helpKeys.article (identifier),
       queryFn: () => getHelpArticle (identifier),
       staleTime: Infinity })
 
+  const language = resolveHelpLanguage (article, localeLanguage (locale))
+
   const query = new URLSearchParams (location.search)
   const requestedEdition = query.get ('edition')
 
@@ -96,7 +99,7 @@ const HelpDetailPage: FC = () => {
 	       : '(最新)'}
 	    </div>)}
 
-	<article className="prose dark:prose-invert mx-auto p-4">
+	<article lang={language} className="prose dark:prose-invert mx-auto p-4">
 	  {Content ? <Content/> : <div>Loading...</div>}
 	</article>
       </MainArea>)

frontend/src/pages/help/HelpDiffPage.tsx

@@ -6,7 +6,9 @@ import PrefetchLink from '@/components/PrefetchLink'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
-import { getHelpDiff } from '@/lib/help'
+import { getHelpArticle, getHelpDiff, resolveHelpLanguage } from '@/lib/help'
+import { localeLanguage } from '@/lib/locales'
+import { useLocale } from '@/lib/useLocale'
 import { helpKeys } from '@/lib/queryKeys'
 import { cn } from '@/lib/utils'
 import NotFound from '@/pages/NotFound'
@@ -17,17 +19,23 @@ const markers = { added: '+ ', removed: '- ', unchanged: '  ' }
 
 
 const HelpDiffPage: FC = () => {
+  const locale = useLocale ()
   const { identifier = '' } = useParams ()
   const [searchParams] = useSearchParams ()
   const from = searchParams.get ('from') ?? ''
   const to = searchParams.get ('to') ?? ''
+  const { data: article } = useQuery ({
+      queryKey: helpKeys.article (identifier),
+      queryFn: () => getHelpArticle (identifier),
+      staleTime: Infinity })
+  const language = resolveHelpLanguage (article, localeLanguage (locale))
   const validParams = Boolean (identifier && from && to)
       && searchParams.getAll ('from').length === 1
       && searchParams.getAll ('to').length === 1
   const { data: diff, isLoading, isError } = useQuery ({
-      enabled: validParams,
-      queryKey: helpKeys.diff (identifier, 'ja', from, to),
-      queryFn: () => getHelpDiff (identifier, 'ja', from, to),
+      enabled: validParams && article != null,
+      queryKey: helpKeys.diff (identifier, language, from, to),
+      queryFn: () => getHelpDiff (identifier, language, from, to),
       staleTime: Infinity })
 
   if (!(validParams) || isError || (!(isLoading) && diff == null))
@@ -55,7 +63,7 @@ const HelpDiffPage: FC = () => {
 		  {diff.to.title} ({to})
 		</PrefetchLink>
 	      </div>
-	      <pre className="mt-4 overflow-x-auto whitespace-pre">
+	      <pre lang={language} className="mt-4 overflow-x-auto whitespace-pre">
 		<code>
 		  {diff.lines.map ((line, index) => {
 		    const colour = cn (

frontend/src/pages/help/HelpListPage.tsx

@@ -15,6 +15,8 @@ import { compareHelpEditions,
 	 getHelpSearchEntries,
 	 helpEditionDate,
 	 normaliseHelpSearchText } from '@/lib/help'
+import { localeLanguage } from '@/lib/locales'
+import { useLocale } from '@/lib/useLocale'
 import { helpKeys } from '@/lib/queryKeys'
 import { LIST_LIMIT_OPTIONS } from '@/lib/settings'
 import { dateString, inputClass } from '@/lib/utils'
@@ -30,6 +32,7 @@ const defaultDirection: Record<HelpOrderField, 'asc' | 'desc'> = {
 
 
 const HelpSearchPage: FC = () => {
+  const language = localeLanguage (useLocale ())
   const [searchParams, setSearchParams] = useSearchParams ()
   const pageValue = searchParams.get ('page') ?? '1'
   const pageNumber = Number (pageValue)
@@ -49,8 +52,8 @@ const HelpSearchPage: FC = () => {
   const [title, setTitle] = useState (conditions.title)
   const [text, setText] = useState (conditions.text)
   const { data: entries = [], isLoading: loading, error } = useQuery ({
-      queryKey: helpKeys.index ('ja'),
-      queryFn: () => getHelpSearchEntries ('ja'),
+      queryKey: helpKeys.index (language),
+      queryFn: () => getHelpSearchEntries (language),
       staleTime: Infinity })
 
   useEffect (() => {

frontend/src/pages/materials/MaterialDetailPage.tsx

@@ -3,6 +3,7 @@ import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useParams } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import TagLink from '@/components/TagLink'
 import WikiBody from '@/components/WikiBody'
 import FieldError from '@/components/common/FieldError'
@@ -28,6 +29,7 @@ type MaterialFormField = 'tag' | 'file' | 'url' | 'exportPaths'
 
 
 const MaterialDetailPage: FC = () => {
+  const locale = useLocale ()
   const { id } = useParams ()
   const qc = useQueryClient ()
 
@@ -40,8 +42,8 @@ const MaterialDetailPage: FC = () => {
       useValidationErrors<MaterialFormField> ()
 
   const { data: material, isError, isLoading } = useQuery ({
-      queryKey: materialsKeys.show (id ?? ''),
-      queryFn:  () => fetchMaterial (id ?? ''),
+      queryKey: materialsKeys.show (id ?? '', locale),
+      queryFn:  () => fetchMaterial (id ?? '', locale),
       enabled:  id != null})
   const materialTitle = material
       ? material.tag?.name ?? `素材 #${ material.id }`
@@ -76,10 +78,10 @@ const MaterialDetailPage: FC = () => {
 	  formData.append ('url', url)
 	formData.append ('export_paths[legacy_drive]', exportPath)
 
-	return await updateMaterial (id ?? '', formData)
+	return await updateMaterial (id ?? '', formData, locale)
       },
       onSuccess: async data => {
-	qc.setQueryData (materialsKeys.show (id ?? ''), data)
+	qc.setQueryData (materialsKeys.show (id ?? '', locale), data)
 	await invalidateMaterialQueries ()
 	toast ({ title: '更新成功!' })
       },

frontend/src/pages/materials/MaterialHistoryPage.tsx

@@ -1,8 +1,9 @@
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
@@ -19,10 +20,11 @@ import type { FC, FormEvent } from 'react'
 
 
 const MaterialHistoryPage: FC = () => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const query = new URLSearchParams (location.search)
 
   const materialId = query.get ('material_id') ?? ''
@@ -47,13 +49,13 @@ const MaterialHistoryPage: FC = () => {
 	  ...(tag && { tag }),
 	  ...(eventType && { eventType }),
 	  page,
-	  limit }),
+	  limit }, locale),
       queryFn: () => fetchMaterialChanges ({
 	  ...(materialId && { materialId }),
 	  ...(tag && { tag }),
 	  ...(eventType && { eventType }),
 	  page,
-	  limit }) })
+	  limit }, locale) })
 
   const versions = data?.versions ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0

frontend/src/pages/materials/MaterialListPage.tsx

@@ -1,8 +1,9 @@
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useMemo, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import nikumaru from '@/assets/fonts/nikumaru.otf'
 import PrefetchLink from '@/components/PrefetchLink'
 import DateTimeField from '@/components/common/DateTimeField'
@@ -210,8 +211,9 @@ const GroupHeading: FC<{
 
 
 const MaterialListPage: FC = () => {
+  const locale = useLocale ()
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const query = useMemo (() => new URLSearchParams (location.search), [location.search])
   const materialFilter = parseMaterialFilter (query.get ('material_filter'), 'present')
 
@@ -277,8 +279,8 @@ const MaterialListPage: FC = () => {
       page,
       limit}
   const { data, isLoading, isError } = useQuery ({
-      queryKey: materialsKeys.index (keys),
-      queryFn:  () => fetchMaterials (keys)})
+      queryKey: materialsKeys.index (keys, locale),
+      queryFn:  () => fetchMaterials (keys, locale)})
   const tagScope = data?.tagScope ?? null
   const materials = data?.materials ?? []
   const groups = data?.groups ?? []

frontend/src/pages/materials/MaterialNewPage.tsx

@@ -1,8 +1,9 @@
 import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import FieldError from '@/components/common/FieldError'
 import Form from '@/components/common/Form'
 import FormField from '@/components/common/FormField'
@@ -13,6 +14,7 @@ import { Button } from '@/components/ui/button'
 import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
 import { createMaterial } from '@/lib/materials'
+import { unlocalisedPath } from '@/lib/locales'
 import { materialsKeys } from '@/lib/queryKeys'
 import { inputClass } from '@/lib/utils'
 import { useValidationErrors } from '@/lib/useValidationErrors'
@@ -23,14 +25,16 @@ type MaterialFormField = 'tag' | 'file' | 'url' | 'exportPaths'
 
 
 const MaterialNewPage: FC = () => {
+  const locale = useLocale ()
   const qc = useQueryClient ()
   const location = useLocation ()
   const query = new URLSearchParams (location.search)
   const tagQuery = query.get ('tag') ?? ''
   const returnToQuery = query.get ('return_to') ?? ''
-  const safeReturnTo = returnToQuery.startsWith ('/materials') ? returnToQuery : ''
+  const safeReturnTo =
+      unlocalisedPath (returnToQuery).startsWith ('/materials') ? returnToQuery : ''
 
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const [file, setFile] = useState<File | null> (null)
   const [filePreview, setFilePreview] = useState ('')
@@ -51,7 +55,7 @@ const MaterialNewPage: FC = () => {
 	  formData.append ('url', url)
 	formData.append ('export_paths[legacy_drive]', exportPath)
 
-	return await createMaterial (formData)
+	return await createMaterial (formData, locale)
       },
       onSuccess: async () => {
 	await qc.invalidateQueries ({ queryKey: materialsKeys.root })

frontend/src/pages/materials/MaterialSyncSuppressionsPage.tsx

@@ -2,6 +2,7 @@ import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
 import { useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 
+import { useLocale } from '@/lib/useLocale'
 import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
@@ -52,6 +53,7 @@ const reasonName = (reason: string): string =>
 
 
 const MaterialSyncSuppressionsPage: FC = () => {
+  const locale = useLocale ()
   const qc = useQueryClient ()
   const [sourceKind, setSourceKind] =
       useState<MaterialSyncSuppressionSourceKind> ('uri')
@@ -61,8 +63,8 @@ const MaterialSyncSuppressionsPage: FC = () => {
   const [reason, setReason] = useState<MaterialSyncSuppressionReason> (REASONS[0])
 
   const { data, isError, isLoading } = useQuery ({
-      queryKey: materialsKeys.suppressions (),
-      queryFn:  fetchMaterialSyncSuppressions})
+      queryKey: materialsKeys.suppressions (locale),
+      queryFn:  () => fetchMaterialSyncSuppressions (locale)})
 
   const createMutation = useMutation ({
       mutationFn: async () =>
@@ -71,9 +73,9 @@ const MaterialSyncSuppressionsPage: FC = () => {
 	      sourceUri,
 	      drivePath,
 	      driveFileId,
-	      reason}),
+	      reason}, locale),
       onSuccess: async () => {
-	await qc.invalidateQueries ({ queryKey: materialsKeys.suppressions () })
+	await qc.invalidateQueries ({ queryKey: materialsKeys.suppressions (locale) })
 	await qc.invalidateQueries ({ queryKey: materialsKeys.root })
 	setSourceUri ('')
 	setDrivePath ('')

frontend/src/pages/posts/PostDetailPage.tsx

@@ -3,6 +3,7 @@ import { motion } from 'framer-motion'
 import { useEffect, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useParams } from 'react-router-dom'
+import { useLocale } from '@/lib/useLocale'
 import PostEditForm from '@/components/PostEditForm'
 import PostEmbed from '@/components/PostEmbed'
 import PostList from '@/components/PostList'
@@ -29,6 +30,7 @@ type Props = { user: User | null }
 
 
 const PostDetailPage: FC<Props> = ({ user }) => {
+  const locale = useLocale ()
   const editable = canEditContent (user)
   const behaviourSettings = useClientBehaviourSettings ()
   const thumbnailMode = behaviourSettings.thumbnailMode ?? 'normal'
@@ -42,12 +44,12 @@ const PostDetailPage: FC<Props> = ({ user }) => {
 	? { duration: .08, ease: 'linear' as const }
 	: { duration: .2, ease: 'easeOut' as const }
   const postId = String (id ?? '')
-  const postKey = postsKeys.show (postId)
+  const postKey = postsKeys.show (postId, locale)
 
   const { data: post, isError: errorFlg, error } = useQuery ({
       enabled:  Boolean (id),
       queryKey: postKey,
-      queryFn:  () => fetchPost (postId) })
+      queryFn:  () => fetchPost (postId, locale) })
 
   const qc = useQueryClient ()
 
@@ -181,7 +183,7 @@ const PostDetailPage: FC<Props> = ({ user }) => {
 			<PostEditForm
 			    post={post}
 			 onSave={newPost => {
-			      qc.setQueryData (postsKeys.show (postId),
+			      qc.setQueryData (postsKeys.show (postId, locale),
 					       (prev: Post | undefined) => newPost ?? prev)
 			      qc.invalidateQueries ({ queryKey: postsKeys.root })
 			      qc.invalidateQueries ({ queryKey: tagsKeys.root })

frontend/src/pages/posts/PostHistoryPage.tsx

@@ -4,6 +4,7 @@ import { useEffect } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useLocation } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import TagLink from '@/components/TagLink'
 import PrefetchLink from '@/components/PrefetchLink'
 import PageTitle from '@/components/common/PageTitle'
@@ -39,6 +40,7 @@ const renderDiff = (diff: { current: string | null; prev: string | null }) => (
 
 
 const PostHistoryPage: FC = () => {
+  const locale = useLocale ()
   const dialogue = useDialogue ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
@@ -61,25 +63,25 @@ const PostHistoryPage: FC = () => {
   const tagQueryId = tagId ?? ''
   const { data: tag } = useQuery ({
       enabled:  Boolean (tagId),
-      queryKey: tagsKeys.show (tagQueryId),
-      queryFn:  () => fetchTag (tagQueryId) })
+      queryKey: tagsKeys.show (tagQueryId, locale),
+      queryFn:  () => fetchTag (tagQueryId, locale) })
 
   const externalTagQueryId = externalTagId ?? ''
   const { data: externalTag } = useQuery ({
       enabled:  Boolean (externalTagId),
-      queryKey: tagsKeys.externalShow (externalTagQueryId),
-      queryFn:  () => fetchExternalTag (externalTagQueryId) })
+      queryKey: tagsKeys.externalShow (externalTagQueryId, locale),
+      queryFn:  () => fetchExternalTag (externalTagQueryId, locale) })
 
   const { data, isLoading: loading } = useQuery ({
       queryKey: postsKeys.changes ({ ...(id && { post: id }),
 				     ...(tagId && { tag: tagId }),
 				     ...(externalTagId && { externalTag: externalTagId }),
-				     page, limit }),
+				     page, limit }, locale),
       queryFn:  () => fetchPostChanges ({ ...(id && { post: id }),
 					  ...(tagId && { tag: tagId }),
 					  ...(externalTagId && {
 					    externalTag: externalTagId }),
-					  page, limit }) })
+					  page, limit }, locale) })
   const changes = data?.versions ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0
   const displayedTag = externalTag ?? tag
@@ -121,7 +123,7 @@ const PostHistoryPage: FC = () => {
       const originalCreatedBefore = change.originalCreatedBefore.current
       await updatePost ({ id, title, tags, duration, parentPostIds,
 			  originalCreatedFrom, originalCreatedBefore },
-		        { force: true })
+		        { force: true }, locale)
 
       qc.invalidateQueries ({ queryKey: postsKeys.root })
       qc.invalidateQueries ({ queryKey: tagsKeys.root })

frontend/src/pages/posts/PostImportReviewPage.tsx

@@ -2,8 +2,9 @@ import { AnimatePresence, motion } from 'framer-motion'
 import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { ChevronRight } from 'lucide-react'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import PrefetchLink from '@/components/PrefetchLink'
@@ -595,10 +596,11 @@ const editableRowDirty = (row: PostImportRow): boolean =>
 
 
 const PostImportReviewPage: FC<Props> = ({ user }) => {
+  const locale = useLocale ()
   const editable = canEditContent (user)
   const dialogue = useDialogue ()
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const existingRowsTransition =
@@ -696,7 +698,7 @@ const PostImportReviewPage: FC<Props> = ({ user }) => {
 
 	  try
 	  {
-	    const preview = await apiGet<PostMetadataResponse> ('/posts/metadata', {
+	    const preview = await apiGet<PostMetadataResponse> ('/posts/metadata', { locale,
 		params: { url: currentRow.url },
 		signal: controller.signal })
 	    if (!(active) || metadataSequenceRef.current !== sequence)
@@ -770,7 +772,7 @@ const PostImportReviewPage: FC<Props> = ({ user }) => {
       active = false
       controllers.forEach (controller => controller.abort ())
     }
-  }, [commitRows, currentRowBySource, location.search, updateRows])
+  }, [commitRows, currentRowBySource, location.search, updateRows, locale])
 
   useEffect (() => {
     if (editingRow == null || resultRepairMode (rowsRef.current) !== 'failed')
@@ -871,12 +873,12 @@ const PostImportReviewPage: FC<Props> = ({ user }) => {
 	  urlChanged
 	  ? mergePreviewRow (
 	      nextRow,
-	      await apiGet<PostMetadataResponse> ('/posts/metadata', {
+	      await apiGet<PostMetadataResponse> ('/posts/metadata', { locale,
 		  params: { url: nextRow.url } }))
 	  : nextRow
       const dryRun = await apiPost<PostMetadataResponse> (
 	  '/posts?dry=1',
-	  buildDryRunFormData (candidateRow))
+	  buildDryRunFormData (candidateRow), { locale })
       const mergedRow = mergeDryRunRow (candidateRow, dryRun)
       const mergedRows = updateRows (currentRows =>
 	  replaceImportRow (currentRows, mergedRow))
@@ -974,7 +976,7 @@ const PostImportReviewPage: FC<Props> = ({ user }) => {
 
       const result = await apiPost<{ results: BulkApiRow[] }>(
 	  '/posts/bulk',
-	  buildBulkFormData ([target]))
+	  buildBulkFormData ([target]), { locale })
       if (result.results.length !== 1)
 	{
 	  updateRows (currentRows => replaceImportRow (currentRows, originalRow))
@@ -1018,7 +1020,7 @@ const PostImportReviewPage: FC<Props> = ({ user }) => {
     {
       const result = await apiPost<{ results: BulkApiRow[] }>(
 	  '/posts/bulk',
-	  buildBulkFormData (latestProcessingRows))
+	  buildBulkFormData (latestProcessingRows), { locale })
       if (result.results.length !== latestProcessingRows.length)
 	{
 	  toast ({ title: '登録結果が不完全でした' })

frontend/src/pages/posts/PostImportSourcePage.tsx

@@ -1,7 +1,8 @@
 import { useEffect, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useNavigate } from 'react-router-dom'
 
+
+import { useLocaleNavigate } from '@/lib/useLocale'
 import FieldError from '@/components/common/FieldError'
 import Form from '@/components/common/Form'
 import FormField from '@/components/common/FormField'
@@ -40,7 +41,7 @@ const SOURCE_ISSUES_ID = 'post-import-source-issues'
 
 const PostImportSourcePage: FC<Props> = ({ user }) => {
   const editable = canEditContent (user)
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const [source, setSource] = useState ('')
   const [loading, setLoading] = useState (false)
   const [sourceIssues, setSourceIssues] = useState<ReturnType<typeof validateImportSource>> ([])

frontend/src/pages/posts/PostListPage.tsx

@@ -1,8 +1,9 @@
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useLayoutEffect, useMemo, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import PostList from '@/components/PostList'
 import PrefetchLink from '@/components/PrefetchLink'
 import TagSidebar from '@/components/TagSidebar'
@@ -33,12 +34,13 @@ const parseLimit = (value: string | null): 20 | 50 | 100 | null => {
 
 
 const PostListPage: FC = () => {
+  const locale = useLocale ()
   const containerRef = useRef<HTMLDivElement | null> (null)
 
   const [wikiPage, setWikiPage] = useState<WikiPage | null> (null)
 
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const query = new URLSearchParams (location.search)
   const tagsQuery = query.get ('tags') ?? ''
   const anyFlg = query.get ('match') === 'any'
@@ -59,8 +61,8 @@ const PostListPage: FC = () => {
       createdFrom: '', createdTo: '', updatedFrom: '', updatedTo: '',
       order } as const
   const { data, isLoading: loading } = useQuery ({
-      queryKey: postsKeys.index (keys),
-      queryFn:  () => fetchPosts (keys) })
+      queryKey: postsKeys.index (keys, locale),
+      queryFn:  () => fetchPosts (keys, locale) })
   const posts = data?.posts ?? []
   const cursor = ''
   const totalPages = data ? Math.ceil (data.count / limit) : 0
@@ -111,14 +113,14 @@ const PostListPage: FC = () => {
       try
       {
 	const tagName = tags[0]
-	setWikiPage (await fetchWikiPageByTitle (tagName, { }))
+	setWikiPage (await fetchWikiPageByTitle (tagName, { }, locale))
       }
       catch
       {
 	;
       }
     }) ()
-  }, [location.search, tags])
+  }, [location.search, tags, locale])
 
   return (
       <div

frontend/src/pages/posts/PostSearchPage.tsx

@@ -2,8 +2,9 @@ import { useQuery } from '@tanstack/react-query'
 import { motion } from 'framer-motion'
 import { useEffect, useMemo, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import SortHeader from '@/components/SortHeader'
 import TagLink from '@/components/TagLink'
@@ -70,6 +71,7 @@ const parseOrder = (value: string | null): FetchPostsOrder | null => {
 
 
 const PostSearchPage: FC = () => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const thumbnailTransition = clientAnimationTransition (
@@ -79,7 +81,7 @@ const PostSearchPage: FC = () => {
   )
   const location = useLocation ()
 
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const query = useMemo (() => new URLSearchParams (location.search),
 			 [location.search])
@@ -130,8 +132,8 @@ const PostSearchPage: FC = () => {
       updatedTo: qUpdatedTo,
       order }
   const { data, isLoading: loading } = useQuery ({
-      queryKey: postsKeys.index (keys),
-      queryFn: () => fetchPosts (keys) })
+      queryKey: postsKeys.index (keys, locale),
+      queryFn: () => fetchPosts (keys, locale) })
   const results = data?.posts ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0
 

frontend/src/pages/tags/NicoTagListPage.tsx

@@ -2,8 +2,9 @@ import { useQuery, useQueryClient } from '@tanstack/react-query'
 import { Check, LoaderCircle, Pencil, X } from 'lucide-react'
 import { Fragment, useEffect, useMemo, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import TagLink from '@/components/TagLink'
 import SortHeader from '@/components/SortHeader'
 import FieldError from '@/components/common/FieldError'
@@ -40,11 +41,12 @@ const setIf = (qs: URLSearchParams, key: string, value: string) => {
 
 
 const NicoTagListPage: FC<Props> = ({ user }) => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const dialogue = useDialogue ()
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const queryClient = useQueryClient ()
   const query = useMemo (() => new URLSearchParams (location.search), [location.search])
 
@@ -66,8 +68,8 @@ const NicoTagListPage: FC<Props> = ({ user }) => {
   const keys = {
       name: qName, linkedTag: qLinkedTag, linkStatus: qLinkStatus, page, limit, order }
   const { data, isError, isLoading: loading } = useQuery ({
-      queryKey: tagsKeys.nicoIndex (keys),
-      queryFn:  () => fetchNicoTags (keys) })
+      queryKey: tagsKeys.nicoIndex (keys, locale),
+      queryFn:  () => fetchNicoTags (keys, locale) })
   const nicoTags = data?.tags ?? []
   const count = data?.count ?? 0
 
@@ -128,7 +130,7 @@ const NicoTagListPage: FC<Props> = ({ user }) => {
     try
     {
       await apiPut<Tag[]> (`/tags/nico/${ id }`, formData,
-			   { headers: { 'Content-Type': 'multipart/form-data' } })
+			   { locale, headers: { 'Content-Type': 'multipart/form-data' } })
       setErrorsByTagId (errors => ({ ...errors, [id]: [] }))
       setEditingId (null)
       await queryClient.invalidateQueries ({ queryKey: tagsKeys.nicoRoot })

frontend/src/pages/tags/TagDetailPage.tsx

@@ -2,6 +2,7 @@ import { useQuery, useQueryClient } from '@tanstack/react-query'
 import { useEffect, useState } from 'react'
 import { useParams } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import TagLink from '@/components/TagLink'
 import FieldError from '@/components/common/FieldError'
 import FormField from '@/components/common/FormField'
@@ -28,13 +29,14 @@ type TagFormField =
 
 
 const TagDetailPage: FC = () => {
+  const locale = useLocale ()
   const { id } = useParams ()
   const tagId = String (id ?? '')
-  const tagKey = tagsKeys.show (tagId)
+  const tagKey = tagsKeys.show (tagId, locale)
 
   const { data: tag, isLoading: loading } = useQuery ({
       queryKey: tagKey,
-      queryFn: () => fetchTag (tagId) })
+      queryFn: () => fetchTag (tagId, locale) })
 
   const [name, setName] = useState ('')
   const [category, setCategory] = useState<Category> ('general')
@@ -60,7 +62,7 @@ const TagDetailPage: FC = () => {
 
     try
     {
-      const data = await apiPut<Tag> (`/tags/${ id }`, formData)
+      const data = await apiPut<Tag> (`/tags/${ id }`, formData, { locale })
 
       setName (data.name)
       setCategory (data.category as Category)

frontend/src/pages/tags/TagHistoryPage.tsx

@@ -3,6 +3,7 @@ import { useEffect } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useLocation } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import PageTitle from '@/components/common/PageTitle'
 import Pagination from '@/components/common/Pagination'
@@ -45,6 +46,7 @@ const renderStateDiff = (diff: { current: string | null; prev: string | null })
 
 
 const TagHistoryPage: FC = () => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const location = useLocation ()
@@ -54,8 +56,8 @@ const TagHistoryPage: FC = () => {
   const limit = Number (query.get ('limit') ?? 20)
 
   const { data, isLoading: loading } = useQuery ({
-      queryKey: tagsKeys.changes ({ ...(id && { id }), page, limit }),
-      queryFn:  () => fetchTagChanges ({ ...(id && { id }), page, limit }) })
+      queryKey: tagsKeys.changes ({ ...(id && { id }), page, limit }, locale),
+      queryFn:  () => fetchTagChanges ({ ...(id && { id }), page, limit }, locale) })
   const changes = data?.versions ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0
 
@@ -210,7 +212,7 @@ const TagHistoryPage: FC = () => {
 					      change.parentTags
 					      .filter (t => t.type !== 'removed')
 					      .map (t => t.tag.name)
-					      .join (' ') })
+					      .join (' ') }, { locale })
 
 				    qc.invalidateQueries ({ queryKey: postsKeys.root })
 				    qc.invalidateQueries ({ queryKey: tagsKeys.root })

frontend/src/pages/tags/TagListPage.tsx

@@ -1,8 +1,9 @@
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useMemo, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import SortHeader from '@/components/SortHeader'
 import TagLink from '@/components/TagLink'
@@ -73,11 +74,12 @@ const parseOrder = (value: string | null): FetchTagsOrder | null => {
 
 
 const TagListPage: FC = () => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const location = useLocation ()
 
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const query = useMemo (() => new URLSearchParams (location.search), [location.search])
 
@@ -129,8 +131,8 @@ const TagListPage: FC = () => {
       updatedTo: qUpdatedTo,
       deprecated: qDeprecated }
   const { data, isLoading: loading } = useQuery ({
-      queryKey: tagsKeys.index (keys),
-      queryFn: () => fetchTags (keys) })
+      queryKey: tagsKeys.index (keys, locale),
+      queryFn: () => fetchTags (keys, locale) })
   const results = data?.tags ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0
 

frontend/src/pages/theatres/TheatreDetailPage.tsx

@@ -3,6 +3,7 @@ import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useParams } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import ErrorScreen from '@/components/ErrorScreen'
 import PostEditForm from '@/components/PostEditForm'
 import PostEmbed from '@/components/PostEmbed'
@@ -215,6 +216,7 @@ type Props = { user: User | null }
 
 
 const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
+  const locale = useLocale ()
   const behaviourSettings = useClientBehaviourSettings ()
   const animationMode = behaviourSettings.animation ?? 'normal'
   const layoutTransition = clientAnimationTransition (
@@ -286,16 +288,16 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
       return
 
     setProgrammes (await apiGet<TheatreProgramme[]> (
-	`/theatres/${ id }/programmes`, { params: { limit: '100' } }))
-  }, [id])
+	`/theatres/${ id }/programmes`, { locale, params: { limit: '100' } }))
+  }, [id, locale])
 
   const refreshWeights = useCallback (async () => {
     if (!(id))
       return
 
     setWeights (await apiGet<TheatrePostSelectionWeights> (
-	`/theatres/${ id }/post_selection_weights`))
-  }, [id])
+	`/theatres/${ id }/post_selection_weights`, { locale }))
+  }, [id, locale])
 
   const advancePost = useCallback (async () => {
     if (!(id))
@@ -304,7 +306,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     setLoading (true)
     try
     {
-      await apiPatch<void> (`/theatres/${ id }/next_post`)
+      await apiPatch<void> (`/theatres/${ id }/next_post`, { }, { locale })
       await refreshProgrammes ()
       await refreshWeights ()
     }
@@ -316,7 +318,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     {
       setLoading (false)
     }
-  }, [id, refreshProgrammes, refreshWeights])
+  }, [id, refreshProgrammes, refreshWeights, locale])
 
   useEffect (() => {
     loadingRef.current = loading
@@ -365,7 +367,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     void (async () => {
       try
       {
-	const data = await apiGet<Theatre> (`/theatres/${ id }`)
+	const data = await apiGet<Theatre> (`/theatres/${ id }`, { locale })
 	if (!(cancelled))
 	  setTheatre (data)
       }
@@ -381,7 +383,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     return () => {
       cancelled = true
     }
-  }, [id, refreshProgrammes, refreshWeights])
+  }, [id, refreshProgrammes, refreshWeights, locale])
 
   useEffect (() => {
     if (!(id))
@@ -400,7 +402,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
       {
 	const newComments = await apiGet<TheatreComment[]> (
 	    `/theatres/${ id }/comments`,
-	    { params: { no_gt: lastCommentNoRef.current, limit: '20' } })
+	    { locale, params: { no_gt: lastCommentNoRef.current, limit: '20' } })
 
 	if (!(cancelled) && newComments.length > 0)
 	  {
@@ -433,7 +435,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
 	  }
 
 	const watchingRequestedAt = performance.now ()
-	const nextInfo = await apiPut<TheatreInfo> (`/theatres/${ id }/watching`)
+	const nextInfo = await apiPut<TheatreInfo> (`/theatres/${ id }/watching`, { }, { locale })
 	if (!(cancelled) && watchingRequestedAt >= theatreInfoReceivedAtRef.current)
 	  applyTheatreInfo (nextInfo)
       }
@@ -454,7 +456,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
       cancelled = true
       clearInterval (interval)
     }
-  }, [applyTheatreInfo, currentPostElapsedMs, id])
+  }, [applyTheatreInfo, currentPostElapsedMs, id, locale])
 
   useEffect (() => {
     if (!(id) || !(theatreInfo.hostFlg) || loadingRef.current || theatreInfo.postId != null)
@@ -487,7 +489,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     void (async () => {
       try
       {
-	const nextPost = await fetchPost (String (theatreInfo.postId))
+	const nextPost = await fetchPost (String (theatreInfo.postId), locale)
 	if (!(cancelled))
 	  setPost (nextPost)
       }
@@ -500,7 +502,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     return () => {
       cancelled = true
     }
-  }, [theatreInfo.postId])
+  }, [theatreInfo.postId, locale])
 
   useEffect (() => {
     void refreshProgrammes ()
@@ -570,9 +572,9 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
       const nextInfo =
 	  theatreInfo.skipVote.voted
 	  ? await apiDelete<TheatreInfo> (
-	      `/theatres/${ id }/skip_vote`, { params: { post_id: post.id } })
+	      `/theatres/${ id }/skip_vote`, { locale, params: { post_id: post.id } })
 	  : await apiPut<TheatreInfo> (
-	      `/theatres/${ id }/skip_vote`, { post_id: post.id })
+	      `/theatres/${ id }/skip_vote`, { post_id: post.id }, { locale })
 
       applyTheatreInfo (nextInfo)
 
@@ -586,7 +588,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     catch (error)
     {
       if (isApiError (error) && error.response?.status === 409)
-	applyTheatreInfo (await apiPut<TheatreInfo> (`/theatres/${ id }/watching`))
+	applyTheatreInfo (await apiPut<TheatreInfo> (`/theatres/${ id }/watching`, { }, { locale }))
 
       console.error (error)
     }
@@ -600,7 +602,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
   const handleDelete = async (commentNo: number) => {
     try
     {
-      await apiDelete (`/theatres/${ id }/comments/${ commentNo }`)
+      await apiDelete (`/theatres/${ id }/comments/${ commentNo }`, { locale })
       setComments (prev => {
 	const rtn = [...prev]
 	const idx = rtn.findIndex (x => x.no === commentNo)
@@ -625,7 +627,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     {
       setSending (true)
       clearValidationErrors ()
-      await apiPost (`/theatres/${ id }/comments`, { content })
+      await apiPost (`/theatres/${ id }/comments`, { content }, { locale })
       setContent ('')
       commentsRef.current?.scrollTo ({
 	  top: 0,

frontend/src/pages/users/SettingPage.tsx

@@ -1,7 +1,8 @@
 import { useCallback, useEffect, useMemo, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useBeforeUnload, useLocation, useNavigate } from 'react-router-dom'
+import { useBeforeUnload, useLocation } from 'react-router-dom'
 
+import { useLocaleNavigate } from '@/lib/useLocale'
 import FieldError from '@/components/common/FieldError'
 import FormField from '@/components/common/FormField'
 import Label from '@/components/common/Label'
@@ -251,7 +252,7 @@ const ensureCompleteThemeSlots = (
 ): UserThemeSlotMap =>
     USER_THEME_SLOT_SELECTIONS.reduce<UserThemeSlotMap> (
 	(complete, selection) => {
-	  const [_, baseTheme, slotNo] = selection.split (':')
+	  const [, baseTheme, slotNo] = selection.split (':')
 	  complete[selection] =
 	      themeSlots[selection]
 	      ?? getDefaultUserThemeSlot (
@@ -267,7 +268,7 @@ const comparableThemeTokensForSelection = (
     themeSlots:  UserThemeSlotMap,
     selection:   UserThemeSlotSelection,
 ): ThemeTokens => {
-  const [_, baseTheme, slotNo] = selection.split (':')
+  const [, baseTheme, slotNo] = selection.split (':')
   const fallback =
       getDefaultUserThemeSlot (
 	  baseTheme as BuiltinThemeId,
@@ -647,7 +648,7 @@ const ThemeSection: FC<ThemeSectionProps> = (
 
 const SettingPage: FC<Props> = ({ user, setUser }) => {
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const dialogue = useDialogue ()
   const { registerUnsavedChangesSource } = useUnsavedChangesGuard ()
 
@@ -811,7 +812,7 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
 
   const discardThemeChanges = () => {
     const selection = displayedThemeSelection.selection
-    const [_, baseTheme, slotNo] = selection.split (':')
+    const [, baseTheme, slotNo] = selection.split (':')
     const nextThemeSlots = {
 	...draftThemeSlots,
 	[selection]: savedThemeSlots[selection]
@@ -894,7 +895,7 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
 	  return
 
 	const selection = displayedThemeSelection.selection
-	const [_, baseTheme, slotNo] = selection.split (':')
+	const [, baseTheme, slotNo] = selection.split (':')
 	themeSlots = {
 	    ...baseState.themeSlots,
 	    [selection]: savedThemeSlots[selection]

frontend/src/pages/wiki/WikiDetailPage.tsx

@@ -1,8 +1,9 @@
 import { useQuery } from '@tanstack/react-query'
 import { useEffect, useMemo } from 'react'
 import { Helmet } from 'react-helmet-async'
-import { useLocation, useNavigate, useParams } from 'react-router-dom'
+import { useLocation, useParams } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import PostList from '@/components/PostList'
 import PrefetchLink from '@/components/PrefetchLink'
 import TagLink from '@/components/TagLink'
@@ -24,11 +25,12 @@ import type { Tag } from '@/types'
 
 
 const WikiDetailPage: FC = () => {
+  const locale = useLocale ()
   const params = useParams ()
   const title = params.title ?? ''
 
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const defaultTag = useMemo (() => ({ name: title, category: 'general' } as Tag), [title])
 
@@ -37,8 +39,8 @@ const WikiDetailPage: FC = () => {
 
   const { data: wikiPage, isLoading: loading, isSuccess, isFetching, error } = useQuery ({
       enabled:  Boolean (title) && !(/^\d+$/.test (title)),
-      queryKey: wikiKeys.show (title, { version }),
-      queryFn:  () => fetchWikiPageByTitle (title, { version }) })
+      queryKey: wikiKeys.show (title, { version }, locale),
+      queryFn:  () => fetchWikiPageByTitle (title, { version }, locale) })
 
   const missingWiki = isSuccess && !(isFetching) && wikiPage == null
   const { data: titleIndex, isPending: lookupPending,
@@ -63,8 +65,8 @@ const WikiDetailPage: FC = () => {
   const { data: tag, isPending: tagPending,
 	  isFetching: tagFetching, error: tagError } = useQuery ({
       enabled:  Boolean (effectiveTitle),
-      queryKey: tagsKeys.show (effectiveTitle),
-      queryFn:  () => fetchTagByName (effectiveTitle) })
+      queryKey: tagsKeys.show (effectiveTitle, locale),
+      queryFn:  () => fetchTagByName (effectiveTitle, locale) })
 
   const keys = {
       tags: effectiveTitle, match: 'all', page: 1, limit: 8, url: '', title: '',
@@ -73,8 +75,8 @@ const WikiDetailPage: FC = () => {
       order: 'original_created_at:desc' } as const
   const { data } = useQuery ({
       enabled:  Boolean (effectiveTitle) && !(version),
-      queryKey: postsKeys.index (keys),
-      queryFn:  () => fetchPosts (keys) })
+      queryKey: postsKeys.index (keys, locale),
+      queryFn:  () => fetchPosts (keys, locale) })
   const posts = data?.posts || []
 
   useEffect (() => {
@@ -97,7 +99,7 @@ const WikiDetailPage: FC = () => {
     void (async () => {
       try
       {
-	const data = await fetchWikiPage (title, { })
+	const data = await fetchWikiPage (title, { }, locale)
 	if (active)
 	  navigate (`/wiki/${ encodeURIComponent (data.title) }`, { replace: true })
       }
@@ -110,7 +112,7 @@ const WikiDetailPage: FC = () => {
     return () => {
       active = false
     }
-  }, [title, navigate])
+  }, [title, navigate, locale])
 
   const displayError = error ?? (missingWiki ? lookupError ?? tagError : null)
   if (displayError != null)

frontend/src/pages/wiki/WikiDiffPage.tsx

@@ -4,6 +4,7 @@ import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useLocation, useParams } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import { SITE_TITLE } from '@/config'
@@ -14,6 +15,7 @@ import type { WikiPageDiff } from '@/types'
 
 
 const WikiDiffPage: FC = () => {
+  const locale = useLocale ()
   const { id } = useParams ()
 
   const location = useLocation ()
@@ -29,9 +31,9 @@ const WikiDiffPage: FC = () => {
 
   useEffect (() => {
     void (async () => {
-      setDiff (await apiGet<WikiPageDiff> (`/wiki/${ id }/diff`, { params: { from, to } }))
+      setDiff (await apiGet<WikiPageDiff> (`/wiki/${ id }/diff`, { locale, params: { from, to } }))
     }) ()
-  }, [from, id, to])
+  }, [from, id, to, locale])
 
   return (
       <MainArea>

frontend/src/pages/wiki/WikiEditPage.tsx

@@ -3,8 +3,9 @@ import MarkdownIt from 'markdown-it'
 import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import MdEditor from 'react-markdown-editor-lite'
-import { useParams, useNavigate } from 'react-router-dom'
+import { useParams } from 'react-router-dom'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import FieldError from '@/components/common/FieldError'
 import FormField from '@/components/common/FormField'
 import MainArea from '@/components/layout/MainArea'
@@ -36,11 +37,12 @@ type WikiFormField = 'title' | 'body'
 
 
 const WikiEditPage: FC<Props> = ({ user }) => {
+  const locale = useLocale ()
   const editable = canEditContent (user)
 
   const { id } = useParams ()
 
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
 
   const qc = useQueryClient ()
 
@@ -60,8 +62,8 @@ const WikiEditPage: FC<Props> = ({ user }) => {
     try
     {
       await apiPut (`/wiki/${ id }`, formData,
-		    { headers: { 'Content-Type': 'multipart/form-data' } })
-      qc.setQueryData (wikiKeys.show (title, { }),
+		    { locale, headers: { 'Content-Type': 'multipart/form-data' } })
+      qc.setQueryData (wikiKeys.show (title, { }, locale),
 		       (prev: WikiPage) => ({ ...prev, title, body }))
       qc.invalidateQueries ({ queryKey: wikiKeys.root })
       toast ({ title: '投稿成功!' })
@@ -80,12 +82,12 @@ const WikiEditPage: FC<Props> = ({ user }) => {
 
     void (async () => {
       setLoading (true)
-      const data = await apiGet<WikiPage> (`/wiki/${ id }`)
+      const data = await apiGet<WikiPage> (`/wiki/${ id }`, { locale })
       setTitle (data.title)
       setBody (data.body)
       setLoading (false)
     }) ()
-  }, [editable, id])
+  }, [editable, id, locale])
 
   if (!(editable))
     return <Forbidden/>

frontend/src/pages/wiki/WikiHistoryPage.tsx

@@ -4,6 +4,7 @@ import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import { useLocation } from 'react-router-dom'
 
+import { useLocale } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
@@ -15,6 +16,7 @@ import type { WikiPageChange } from '@/types'
 
 
 const WikiHistoryPage: FC = () => {
+  const locale = useLocale ()
   const [changes, setChanges] = useState<WikiPageChange[]> ([])
 
   const location = useLocation ()
@@ -23,9 +25,9 @@ const WikiHistoryPage: FC = () => {
 
   useEffect (() => {
     void (async () => {
-      setChanges (await apiGet<WikiPageChange[]> ('/wiki/changes', { params: id ? { id } : { } }))
+      setChanges (await apiGet<WikiPageChange[]> ('/wiki/changes', { locale, params: id ? { id } : { } }))
     }) ()
-  }, [id, location.search])
+  }, [id, location.search, locale])
 
   return (
       <MainArea>

frontend/src/pages/wiki/WikiNewPage.tsx

@@ -3,9 +3,10 @@ import MarkdownIt from 'markdown-it'
 import { useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 import MdEditor from 'react-markdown-editor-lite'
-import { useLocation, useNavigate } from 'react-router-dom'
+import { useLocation } from 'react-router-dom'
 import 'react-markdown-editor-lite/lib/index.css'
 
+import { useLocale, useLocaleNavigate } from '@/lib/useLocale'
 import FieldError from '@/components/common/FieldError'
 import FormField from '@/components/common/FormField'
 import MainArea from '@/components/layout/MainArea'
@@ -35,10 +36,11 @@ type WikiFormField = 'title' | 'body'
 
 
 const WikiNewPage: FC<Props> = ({ user }) => {
+  const locale = useLocale ()
   const editable = canEditContent (user)
 
   const location = useLocation ()
-  const navigate = useNavigate ()
+  const navigate = useLocaleNavigate ()
   const qc = useQueryClient ()
 
   const query = new URLSearchParams (location.search)
@@ -59,9 +61,9 @@ const WikiNewPage: FC<Props> = ({ user }) => {
     try
     {
       const data = await apiPost<WikiPage> ('/wiki', formData,
-				  { headers: { 'Content-Type': 'multipart/form-data' } })
+				  { locale, headers: { 'Content-Type': 'multipart/form-data' } })
       await qc.cancelQueries ({ queryKey: wikiKeys.root })
-      qc.setQueryData (wikiKeys.show (data.title, { }), data)
+      qc.setQueryData (wikiKeys.show (data.title, { }, locale), data)
       await Promise.all ([
 	  qc.invalidateQueries ({ queryKey: wikiKeys.root }),
 	  qc.invalidateQueries ({ queryKey: tagsKeys.root })])

frontend/src/pages/wiki/WikiSearchPage.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from 'react'
 import { Helmet } from 'react-helmet-async'
 
+import { useLocale } from '@/lib/useLocale'
 import PrefetchLink from '@/components/PrefetchLink'
 import FormField from '@/components/common/FormField'
 import PageTitle from '@/components/common/PageTitle'
@@ -15,12 +16,13 @@ import type { WikiPage } from '@/types'
 
 
 const WikiSearchPage: FC = () => {
+  const locale = useLocale ()
   const [title, setTitle] = useState ('')
   const [text, setText] = useState ('')
   const [results, setResults] = useState<WikiPage[]> ([])
 
   const search = async () => {
-    setResults (await apiGet ('/wiki', { params: { title } }))
+    setResults (await apiGet ('/wiki', { locale, params: { title } }))
   }
 
   const handleSearch = (ev: FormEvent) => {
@@ -30,9 +32,9 @@ const WikiSearchPage: FC = () => {
 
   useEffect (() => {
     void (async () => {
-      setResults (await apiGet ('/wiki', { params: { title: '' } }))
+      setResults (await apiGet ('/wiki', { locale, params: { title: '' } }))
     }) ()
-  }, [])
+  }, [locale])
 
   return (
       <MainArea>

frontend/vite.config.ts

@@ -16,7 +16,7 @@ if (typeof originalTransform === 'function')
 	function (code, id)
 	{
 	  return (/[?&](raw|url)(?:&|$)/.test (id)
-		  ? null
+		  ? Promise.resolve (undefined)
 		  : originalTransform.call (this, code, id))
 	}
   }