diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index 4ca7a17..6b1c633 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -7,6 +7,8 @@ class PostsController < ApplicationController
   end
 
   def index
+    locale = resolve_locale!
+
     url = params[:url].presence
     title = params[:title].presence
     original_created_from = params[:original_created_from].presence
@@ -46,7 +48,7 @@ class PostsController < ApplicationController
         'COALESCE(pt_max.max_updated_at, posts.updated_at))'
 
     q =
-        filtered_posts
+        filtered_posts(locale)
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
         .preload(:external_tags, :uploaded_user, :parents, :children,
@@ -102,8 +104,10 @@ class PostsController < ApplicationController
   end
 
   def random
+    locale = resolve_locale!
+
     post =
-        filtered_posts
+        filtered_posts(locale)
         .preload(:uploaded_user, :parents, :children,
                  post_tags: [:sections, { tag: [:deerjikists, :materials,
                                                 { tag_name: :wiki_page }] }])
@@ -394,19 +398,19 @@ class PostsController < ApplicationController
 
   private
 
-  def filtered_posts
+  def filtered_posts locale
     tag_names = params[:tags].to_s.split
     match_type = params[:match]
     if tag_names.present?
-      filter_posts_by_tags(tag_names, match_type)
+      filter_posts_by_tags(locale, tag_names, match_type)
     else
       Post.all
     end
   end
 
-  def filter_posts_by_tags tag_names, match_type
+  def filter_posts_by_tags locale, tag_names, match_type
     literals = tag_names.map do |raw_name|
-      { name: TagName.canonicalise(raw_name.sub(/\Anot:/i, '')).first,
+      { name: TagName.canonicalise(locale, raw_name.sub(/\Anot:/i, '')).first,
         negative: raw_name.downcase.start_with?('not:') }
     end
 
diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index e906737..d8cbcd4 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -430,7 +430,7 @@ class TagsController < ApplicationController
       else
         tag.update!(category:, deprecated_at: deprecated ? Time.current : nil)
       end
-      rename_tag_name!(tag, name) if name_changed
+      rename_tag_name!(locale, tag, name) if name_changed
 
       alias_names << old_name if name_changed
       alias_names.delete(name)
@@ -701,13 +701,13 @@ class TagsController < ApplicationController
     false
   end
 
-  def rename_tag_name! tag, name
-    return if name == tag.name
+  def rename_tag_name! locale, tag, name
+    return if name == tag.name(locale.language_code)
 
-    current_tag_name = tag.tag_name
-    target_tag_name = TagName.find_by(name:)
+    current_tag_name = tag.tag_name(locale.language_code)
+    target_tag_name = TagName.find_by(language_code: locale.language_code, name:)
 
-    if target_tag_name.nil?
+    unless target_tag_name
       current_tag_name.update!(name:)
       return
     end
@@ -722,19 +722,21 @@ class TagsController < ApplicationController
       TagVersioning.ensure_snapshot!(old_owner_tag, created_by_user: current_user)
     end
 
-    promoted_tag_name.update!(canonical: nil)
+    promoted_tag_name.update!(canonical_id: nil, primary_flg: true)
 
-    TagName.where(canonical_id: current_tag_name.id)
+    TagName.where(tag:,
+                  language_code: current_tag_name.language.language_code,
+                  primary_flg: false)
            .where.not(id: promoted_tag_name.id)
            .find_each do |alias_tag_name|
-      alias_tag_name.update!(canonical: promoted_tag_name)
+      alias_tag_name.update!(canonical_id: promoted_tag_name, primary_flg: false)
     end
 
-    current_tag_name.wiki_page&.update!(tag_name: promoted_tag_name)
-    tag.update!(tag_name: promoted_tag_name)
+    current_tag_name.wiki_page&.update!(tag_name_id: promoted_tag_name.id)
+    tag.update!(tag_name_id: promoted_tag_name.id)
     current_tag_name.association(:wiki_page).reset
     current_tag_name.association(:tag).reset
-    current_tag_name.reload.update!(canonical: promoted_tag_name)
+    current_tag_name.reload.update!(canonical_id: promoted_tag_name, primary_flg: false)
 
     return unless old_owner_tag && old_owner_tag != tag
 
@@ -766,12 +768,12 @@ class TagsController < ApplicationController
     current_aliases.each do |alias_tag_name|
       next if alias_names.include?(alias_tag_name.name)
 
-      alias_tag_name.update!(canonical: nil)
+      alias_tag_name.update!(canonical_id: nil)
     end
 
     alias_names.each do |alias_name|
       alias_tag_name = TagName.find_or_create_by!(name: alias_name)
-      alias_tag_name.update!(canonical: tag.tag_name)
+      alias_tag_name.update!(canonical_id: tag.tag_name.id)
     end
 
     affected_tags.compact.uniq.each do |affected_tag|
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 2648097..a8cdfd6 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -51,12 +51,10 @@ class Tag < ApplicationRecord
   has_many :tag_versions
 
   has_many :tag_names
-  belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
 
   attribute :version_no, :integer, default: 1
 
-  delegate :name, to: :tag_name, allow_nil: true
   validates :tag_name, presence: true
 
   enum :category, deerjikist:  'deerjikist',
@@ -84,9 +82,8 @@ class Tag < ApplicationRecord
       'mtr:'        => :material,
       'meta:'       => :meta }.freeze
 
-  def name= val
-    (self.tag_name ||= build_tag_name).name = val
-  end
+  def tag_name(language_code) = tag_names.find_by(language_code:, primary_flg: true)
+  def name(language_code) = tag_name(language_code)&.name
 
   def deprecated? = deprecated_at?
 
@@ -276,7 +273,7 @@ class Tag < ApplicationRecord
         end
 
         TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
-        source_tag.tag_names.update_all(tag_id: source_tag.id, primary_key: false,
+        source_tag.tag_names.update_all(tag_id: target_tag.id, primary_flg: false,
                                         updated_at: Time.current)
         source_tag.destroy!
 
diff --git a/backend/app/models/tag_name.rb b/backend/app/models/tag_name.rb
index 91081ee..046cc27 100644
--- a/backend/app/models/tag_name.rb
+++ b/backend/app/models/tag_name.rb
@@ -13,8 +13,8 @@ class TagName < ApplicationRecord
 
   def primary? = primary_flg
 
-  def canonical = TagName.find_by(language:, tag:, primary_flg: true)
-  def aliases = TagName.where(language:, tag:, primary_flg: false)
+  def canonical = TagName.find_by(language_code:, tag_id:, primary_flg: true)
+  def aliases = TagName.where(language_code:, tag_id:, primary_flg: false)
 
   def self.canonicalise locale, names
     names = Array(names).map { |n| n.to_s.strip }.reject(&:blank?)
@@ -23,7 +23,7 @@ class TagName < ApplicationRecord
     tns = TagName.where(language_code: locale.language_code, name: names).index_by(&:name)
 
     names.map { |name|
-      if tns[name].primary_flg
+      if !(tns[name]) || tns[name].primary?
         name
       else
         TagName.find_by(language_code: locale.language_code,
diff --git a/backend/app/services/post_create_plan.rb b/backend/app/services/post_create_plan.rb
index 6a0f9e6..e1279a1 100644
--- a/backend/app/services/post_create_plan.rb
+++ b/backend/app/services/post_create_plan.rb
@@ -10,7 +10,7 @@ class PostCreatePlan
 
   def build! locale
     direct_tag_specs, tag_sections = parse_direct_tag_specs(locale)
-    default_tag_specs = build_default_tag_specs(direct_tag_specs)
+    default_tag_specs = build_default_tag_specs(locale, direct_tag_specs)
     snapshot_tag_specs = merge_tag_specs(direct_tag_specs + default_tag_specs)
     preload_existing_tags_by_name!(snapshot_tag_specs.map { _1[:name] })
     validate_new_tag_specs!(snapshot_tag_specs)
@@ -263,12 +263,11 @@ class PostCreatePlan
   end
 
   def resolved_tag_name locale, name
-    tag_name = TagName.find_by!(language_code: locale.language_code, name:)
-    unless tag_name.primary_flg
-      tag_name = TagName.find_by!(language_code: locale.language_code,
-                                  tag_id: tag_name.tag_id,
-                                  primary_flg: true)
-    end
-    tag_name.name
+    tag_name = TagName.find_by(language_code: locale.language_code, name:)
+    return name if !(tag_name) || tag_name.primary?
+
+    TagName.find_by!(language_code: locale.language_code,
+                     tag_id: tag_name.tag_id,
+                     primary_flg: true).name
   end
 end
diff --git a/backend/app/services/post_creator.rb b/backend/app/services/post_creator.rb
index b200901..f328068 100644
--- a/backend/app/services/post_creator.rb
+++ b/backend/app/services/post_creator.rb
@@ -74,13 +74,13 @@ class PostCreator
             normalised_parent_post_ids: @attributes[:normalised_parent_post_ids] || [],
             video_ms: @attributes[:video_ms] }
       else
-        build_materialised_plan
+        build_materialised_plan(locale)
       end
     end
   end
 
-  def build_materialised_plan
-    plan = PostCreatePlan.new(attributes: @attributes).build!
+  def build_materialised_plan locale
+    plan = PostCreatePlan.new(attributes: @attributes).build!(locale)
     snapshot_tags = materialise_tags(plan[:snapshot_tag_specs] || [])
     post_tags = materialise_tags(plan[:post_tag_specs] || [])
     {
diff --git a/backend/spec/factories/tags.rb b/backend/spec/factories/tags.rb
index a9df735..b362e23 100644
--- a/backend/spec/factories/tags.rb
+++ b/backend/spec/factories/tags.rb
@@ -1,20 +1,29 @@
 FactoryBot.define do
   factory :tag do
     transient do
-      name { nil }
+      primary_name { nil }
+      primary_tag_name do
+        attributes = primary_name.nil? ? { } : { name: primary_name }
+        build(:tag_name, **attributes)
+      end
     end
 
     category { :general }
     post_count { 0 }
 
-    association :tag_name
-
     after(:build) do |tag, evaluator|
-      tag.name = evaluator.name if evaluator.name.present?
+      evaluator.primary_tag_name.primary_flg = true
+      tag.tag_names.target << evaluator.primary_tag_name
     end
 
-    after(:create) do |tag|
-      tag.tag_name.update!(tag:) if tag.tag_name.tag_id != tag.id
+    before(:create) do |tag, evaluator|
+      evaluator.primary_tag_name.save!
+      # Compatibility write only; ownership is established through tag_id below.
+      tag.tag_name_id = evaluator.primary_tag_name.id
+    end
+
+    after(:create) do |tag, evaluator|
+      evaluator.primary_tag_name.update!(tag:, primary_flg: true)
     end
   end
 end
diff --git a/backend/spec/models/locale_spec.rb b/backend/spec/models/locale_spec.rb
index a3b328e..54304bf 100644
--- a/backend/spec/models/locale_spec.rb
+++ b/backend/spec/models/locale_spec.rb
@@ -13,17 +13,19 @@ RSpec.describe Locale, type: :model do
   end
 
   it 'generates a primary name for every existing tag using the name generator' do
-    first = create(:tag, name: 'first_existing_tag')
-    second = create(:tag, name: 'second_existing_tag')
+    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
     allow(TagName).to receive(:generate_name)
-        .with(kind_of(described_class), first, first.name)
+        .with(kind_of(described_class), first, primary_tag_name_for(first, 'ja').name)
         .and_return('name_from_generator')
 
     locale = create_french_locale!
 
-    expect(TagName).to have_received(:generate_name).with(locale, first, first.name)
-    expect(TagName).to have_received(:generate_name).with(locale, second, second.name)
+    expect(TagName).to have_received(:generate_name)
+        .with(locale, first, primary_tag_name_for(first, 'ja').name)
+    expect(TagName).to have_received(:generate_name)
+        .with(locale, second, primary_tag_name_for(second, 'ja').name)
     expect(first.tag_names.find_by!(language_code: 'fr', primary_flg: true))
         .to have_attributes(name: 'name_from_generator', tag_id: first.id,
                             language_code: 'fr', script_code: 'Latn',
@@ -37,7 +39,7 @@ RSpec.describe Locale, type: :model do
 
   it 'does not duplicate an existing primary name in the new language' do
     prepare_french_reference!
-    tag = create(:tag, name: 'already_named')
+    tag = create(:tag, primary_name: 'already_named')
     existing = create(:tag_name, name: 'nom_existant', tag:,
                                  language_code: 'fr', script_code: 'Latn')
 
@@ -51,7 +53,7 @@ RSpec.describe Locale, type: :model do
 
   it 'creates a primary name when the language has only an alias' do
     prepare_french_reference!
-    tag = create(:tag, name: 'alias_only_tag')
+    tag = create(:tag, primary_name: 'alias_only_tag')
     alias_name = create(:tag_name, :alias, name: 'alias_fr',
                                           tag:,
                                           language_code: 'fr', script_code: 'Latn')
diff --git a/backend/spec/models/material_export_item_spec.rb b/backend/spec/models/material_export_item_spec.rb
index 07e08f3..19734a0 100644
--- a/backend/spec/models/material_export_item_spec.rb
+++ b/backend/spec/models/material_export_item_spec.rb
@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe MaterialExportItem, type: :model do
   let(:user) { create(:user, :member) }
-  let(:tag) { create(:tag, name: 'export_item', category: :material) }
+  let(:tag) { create(:tag, primary_name: 'export_item', category: :material) }
   let(:material) do
     Material.create!(tag:, url: 'https://example.com/material',
                      created_by_user: user, updated_by_user: user)
diff --git a/backend/spec/models/post_spec.rb b/backend/spec/models/post_spec.rb
index ea5646e..f12d90b 100644
--- a/backend/spec/models/post_spec.rb
+++ b/backend/spec/models/post_spec.rb
@@ -18,7 +18,7 @@ RSpec.describe Post, type: :model do
       expect(post.snapshot_tags_json).to eq([
           { 'tag_id' => tag.id,
             'version_no' => tag.version_no,
-            'name' => tag.name,
+            'name' => primary_tag_name_for(tag, 'ja').name,
             'category' => tag.category,
             'sections' => [{ 'begin_ms' => 2000, 'end_ms' => nil }] },
           { 'external_tag_id' => external.id }])
diff --git a/backend/spec/models/post_version_spec.rb b/backend/spec/models/post_version_spec.rb
index ccbb20f..f29b530 100644
--- a/backend/spec/models/post_version_spec.rb
+++ b/backend/spec/models/post_version_spec.rb
@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe PostVersion, type: :model do
   let!(:tag_name) { create(:tag_name, name: 'post_version_spec_tag') }
-  let!(:tag) { create(:tag, tag_name: tag_name, category: :general) }
+  let!(:tag) { create(:tag, primary_tag_name: tag_name, category: :general) }
 
   let!(:post_record) do
     Post.create!(title: 'spec post', url: 'https://example.com/post-version-spec').tap do |post|
diff --git a/backend/spec/models/tag_implication_spec.rb b/backend/spec/models/tag_implication_spec.rb
index a29cc1c..9034d2e 100644
--- a/backend/spec/models/tag_implication_spec.rb
+++ b/backend/spec/models/tag_implication_spec.rb
@@ -2,8 +2,8 @@ require 'rails_helper'
 
 RSpec.describe TagImplication, type: :model do
   it 'rejects a parent tag that would create a cycle' do
-    child = create(:tag, name: 'tag_implication_cycle_child')
-    parent = create(:tag, name: 'tag_implication_cycle_parent')
+    child = create(:tag, primary_name: 'tag_implication_cycle_child')
+    parent = create(:tag, primary_name: 'tag_implication_cycle_parent')
 
     described_class.create!(tag: child, parent_tag: parent)
 
@@ -17,9 +17,9 @@ RSpec.describe TagImplication, type: :model do
   end
 
   it 'terminates even when existing data already contains a cycle' do
-    child = create(:tag, name: 'tag_implication_existing_cycle_child')
-    parent = create(:tag, name: 'tag_implication_existing_cycle_parent')
-    ancestor = create(:tag, name: 'tag_implication_existing_cycle_ancestor')
+    child = create(:tag, primary_name: 'tag_implication_existing_cycle_child')
+    parent = create(:tag, primary_name: 'tag_implication_existing_cycle_parent')
+    ancestor = create(:tag, primary_name: 'tag_implication_existing_cycle_ancestor')
 
     described_class.create!(tag: parent, parent_tag: ancestor)
     described_class.insert_all!(
diff --git a/backend/spec/models/tag_name_sanitisation_rule_spec.rb b/backend/spec/models/tag_name_sanitisation_rule_spec.rb
index 50d0a10..558869e 100644
--- a/backend/spec/models/tag_name_sanitisation_rule_spec.rb
+++ b/backend/spec/models/tag_name_sanitisation_rule_spec.rb
@@ -50,17 +50,17 @@ RSpec.describe TagNameSanitisationRule, type: :model do
       end
 
       it 'renames without merging tags or deleting either language identity' do
-        japanese = create(:tag, name: 'foobar')
+        japanese = create(:tag, primary_name: 'foobar')
         english_name = create(:tag_name, name: 'temporary',
                                         language_code: 'en', script_code: 'Latn')
-        english = create(:tag, tag_name: english_name)
+        english = create(:tag, primary_tag_name: english_name)
         english_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(japanese.reload.tag_name).to have_attributes(
+        expect(primary_tag_name_for(japanese.reload, 'ja')).to have_attributes(
             name: 'foobar', language_code: 'ja', tag_id: japanese.id)
         expect(english.reload.tag_name_id).to eq(english_name.id)
       end
@@ -96,15 +96,19 @@ RSpec.describe TagNameSanitisationRule, type: :model do
 
     context 'when the source tag_name has a tag and the existing one has no tag' do
       let!(:existing) { create(:tag_name, name: 'foobar') }
-      let!(:source_tag) { create(:tag, name: 'tmp', category: :general) }
-      let!(:source_tag_name_id) { source_tag.tag_name_id }
+      let!(:source_tag) { create(:tag, primary_name: 'tmp', category: :general) }
+      let!(:source_tag_name_id) { primary_tag_name_for(source_tag, 'ja').id }
 
       before do
-        source_tag.tag_name.update_columns(name: 'foo_bar', updated_at: Time.current)
+        primary_tag_name_for(source_tag, 'ja').update_columns(
+            name: 'foo_bar', updated_at: Time.current)
       end
 
       it 'moves the tag to the existing tag_name' do
         described_class.apply!
+        expect(existing.reload).to have_attributes(
+            tag_id: source_tag.id, language_code: 'ja', primary_flg: true)
+        expect(primary_tag_name_for(source_tag, 'ja')).to eq(existing)
         expect(source_tag.reload.tag_name_id).to eq(existing.id)
         expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
       end
@@ -112,6 +116,7 @@ RSpec.describe TagNameSanitisationRule, type: :model do
 
     context 'when the sanitised name is an alias of an existing tag' do
       let!(:existing_tag) { create(:tag) }
+      let!(:existing_primary) { primary_tag_name_for(existing_tag, 'ja') }
       let!(:alias_name) do
         create(:tag_name, :alias, name: 'foobar', tag: existing_tag)
       end
@@ -129,17 +134,17 @@ RSpec.describe TagNameSanitisationRule, type: :model do
             tag_id: existing_tag.id, language_code: 'ja', primary_flg: false)
         expect(TagName.find_by!(tag_id: existing_tag.id,
                                 language_code: 'ja', primary_flg: true))
-            .to eq(existing_tag.tag_name)
+            .to eq(existing_primary)
         expect(Tag.find(existing_tag.id)).to eq(existing_tag)
       end
     end
 
     context 'when both source and existing tag_names have tags' do
       let!(:existing_tn) { create(:tag_name, name: 'foobar') }
-      let!(:existing_tag) { create(:tag, tag_name: existing_tn, category: :general) }
+      let!(:existing_tag) { create(:tag, primary_tag_name: existing_tn, category: :general) }
 
       let!(:source_tn) { create(:tag_name, name: 'tmp') }
-      let!(:source_tag) { create(:tag, tag_name: source_tn, category: :general) }
+      let!(:source_tag) { create(:tag, primary_tag_name: source_tn, category: :general) }
       let!(:source_tag_name_id) { source_tn.id }
 
       before do
@@ -155,7 +160,7 @@ RSpec.describe TagNameSanitisationRule, type: :model do
         expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
         expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
         expect(post.reload.tags).to contain_exactly(existing_tag)
-        expect(existing_tag.reload.name).to eq('foobar')
+        expect(primary_tag_name_for(existing_tag.reload, 'ja').name).to eq('foobar')
       end
     end
   end
diff --git a/backend/spec/models/tag_name_spec.rb b/backend/spec/models/tag_name_spec.rb
index b8e251f..37107cc 100644
--- a/backend/spec/models/tag_name_spec.rb
+++ b/backend/spec/models/tag_name_spec.rb
@@ -28,16 +28,33 @@ RSpec.describe TagName, type: :model do
   end
 
   it 'persists both sides of the representative tag factory association' do
-    tag = create(:tag)
+    name = create(:tag_name)
+    tag = create(:tag, primary_tag_name: name)
 
-    expect(tag.reload.tag_name.reload.tag_id).to eq(tag.id)
-    expect(tag.tag_name.tag).to eq(tag)
+    expect(name.reload).to have_attributes(
+        tag_id: tag.id, language_code: 'ja', script_code: 'Jpan', primary_flg: true)
+    expect(name.tag).to eq(tag)
     expect(described_class.find_by!(tag_id: tag.id, language_code: 'ja', primary_flg: true))
-        .to eq(tag.tag_name)
+        .to eq(name)
+    expect(tag.reload.tag_name_id).to eq(name.id)
+  end
+
+  it 'builds a primary name without persisting the tag or its name' do
+    tag = nil
+
+    expect {
+      tag = build(:tag, primary_name: 'built_primary')
+    }.to change(Tag, :count).by(0).and change(described_class, :count).by(0)
+
+    expect(tag).to be_new_record
+    expect(tag.tag_names.target).to contain_exactly(have_attributes(
+        name: 'built_primary', language_code: 'ja', script_code: 'Jpan',
+        primary_flg: true))
   end
 
   it 'persists an alias owned by the same tag as its primary name' do
-    tag = create(:tag)
+    primary_name = create(:tag_name)
+    tag = create(:tag, primary_tag_name: primary_name)
     alias_name = create(:tag_name, :alias, name: 'valid_alias', tag:)
 
     expect(alias_name.reload).to have_attributes(
@@ -46,12 +63,12 @@ RSpec.describe TagName, type: :model do
     primary = described_class.find_by!(tag_id: alias_name.tag_id,
                                        language_code: alias_name.language_code,
                                        primary_flg: true)
-    expect(primary).to eq(tag.tag_name)
+    expect(primary).to eq(primary_name)
   end
 
   describe '.canonicalise' do
     it 'resolves the primary name of the alias language on the same tag' do
-      tag = create(:tag, name: '日本語正本')
+      tag = create(:tag, primary_name: '日本語正本')
       english_primary = create(:tag_name, tag:, name: 'english_primary',
                                           language_code: 'en', script_code: 'Latn')
       english_alias = create(:tag_name, :alias, tag:, name: 'english_alias',
@@ -69,10 +86,10 @@ RSpec.describe TagName, type: :model do
     end
 
     it 'resolves only aliases in the requested language' do
-      japanese = create(:tag, name: 'japanese_canonical')
+      japanese = create(:tag, primary_name: 'japanese_canonical')
       english_name = create(:tag_name, name: 'english_canonical',
                                       language_code: 'en', script_code: 'Latn')
-      english = create(:tag, tag_name: english_name)
+      english = create(:tag, primary_tag_name: english_name)
       japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                  tag: japanese)
       english_alias = create(:tag_name, :alias, name: 'shared_alias',
diff --git a/backend/spec/models/tag_spec.rb b/backend/spec/models/tag_spec.rb
index 13feb00..d900053 100644
--- a/backend/spec/models/tag_spec.rb
+++ b/backend/spec/models/tag_spec.rb
@@ -4,7 +4,7 @@ RSpec.describe Tag, type: :model do
   describe 'external tag separation' do
     ['nico:reserved', 'NiCo:reserved'].each do |name|
       it "rejects the reserved prefix #{ name } for internal tags" do
-        tag = build(:tag, name:)
+        tag = build(:tag, primary_name: name)
 
         expect(tag).to be_invalid
         expect(tag.errors[:name]).to be_present
@@ -32,10 +32,10 @@ RSpec.describe Tag, type: :model do
           { code: 'en', language_code: 'en', script_code: 'Latn',
             name: 'English', created_at: Time.current }]) unless Locale.exists?(code: 'en')
       english_locale = Locale.find('en')
-      japanese = create(:tag, name: 'japanese_canonical')
+      japanese = create(:tag, primary_name: 'japanese_canonical')
       english_name = create(:tag_name, name: 'english_canonical',
                                       language_code: 'en', script_code: 'Latn')
-      english = create(:tag, tag_name: english_name)
+      english = create(:tag, primary_tag_name: english_name)
       japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                  tag: japanese)
       english_alias = create(:tag_name, :alias, name: 'shared_alias',
@@ -59,16 +59,16 @@ RSpec.describe Tag, type: :model do
     it 'rejects deprecated tags when deny_deprecated is enabled' do
       tag_name = create(:tag_name, name: 'normalise deprecated tag')
       deprecated_tag = create(:tag,
-          tag_name:,
+          primary_tag_name: tag_name,
           category: :general,
           deprecated_at: 1.day.from_now)
 
       expect {
         described_class.normalise_tags!(Locale.nipponese,
-            [deprecated_tag.name],
+            [primary_tag_name_for(deprecated_tag, 'ja').name],
             deny_deprecated: true)
       }.to raise_error(Tag::DeprecatedTagNormalisationError) { |error|
-        expect(error.tag_names).to eq([deprecated_tag.name])
+        expect(error.tag_names).to eq([primary_tag_name_for(deprecated_tag, 'ja').name])
       }
     end
 
@@ -85,7 +85,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[1:00-]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, nil]])
     end
 
@@ -94,7 +96,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[-1:00]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[0, 60_000]])
     end
 
@@ -103,7 +107,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[-]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections)[tag.id]).to be_nil
     end
 
@@ -112,7 +118,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[0:00-]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections)[tag.id]).to be_nil
     end
 
@@ -121,7 +129,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[1:00-1:00]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, 60_001]])
     end
 
@@ -130,7 +140,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[2:00-1:00]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, 120_000]])
     end
 
@@ -139,7 +151,9 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[1:00-][2:00-3:00]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, nil]])
     end
 
@@ -148,25 +162,23 @@ RSpec.describe Tag, type: :model do
           ['伊地知ニジカ[1:00-3:00][3:00-]'],
           with_sections: true)
 
-      tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
+      tag = result.fetch(:tags).find do |candidate|
+        primary_tag_name_for(candidate, 'ja').name == '伊地知ニジカ'
+      end
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, nil]])
     end
   end
 
   describe '.expand_parent_tags' do
     it 'expands through multiple deprecated parents to an active ancestor' do
-      child = create(:tag, name: 'expand_child')
-      deprecated_parent = create(
-        :tag,
-        name: 'expand_deprecated_parent',
-        deprecated_at: Time.current
-      )
-      deprecated_grandparent = create(
-        :tag,
-        name: 'expand_deprecated_grandparent',
-        deprecated_at: Time.current
-      )
-      active_ancestor = create(:tag, name: 'expand_active_ancestor')
+      child = create(:tag, primary_name: 'expand_child')
+      deprecated_parent = create(:tag,
+          primary_name: 'expand_deprecated_parent',
+          deprecated_at: Time.current)
+      deprecated_grandparent = create(:tag,
+          primary_name: 'expand_deprecated_grandparent',
+          deprecated_at: Time.current)
+      active_ancestor = create(:tag, primary_name: 'expand_active_ancestor')
       TagImplication.create!(tag: child, parent_tag: deprecated_parent)
       TagImplication.create!(tag: deprecated_parent, parent_tag: deprecated_grandparent)
       TagImplication.create!(tag: deprecated_grandparent, parent_tag: active_ancestor)
@@ -183,8 +195,8 @@ RSpec.describe Tag, type: :model do
     end
 
     it 'terminates when implications contain a cycle' do
-      first = create(:tag, name: 'expand_cycle_first')
-      second = create(:tag, name: 'expand_cycle_second')
+      first = create(:tag, primary_name: 'expand_cycle_first')
+      second = create(:tag, primary_name: 'expand_cycle_second')
       TagImplication.create!(tag: first, parent_tag: second)
       now = Time.current
       TagImplication.insert_all!(
@@ -210,7 +222,7 @@ RSpec.describe Tag, type: :model do
         tag = described_class.find_or_create_by_tag_name!(
             locale, 'english_name', category: :character)
 
-        expect(tag.tag_name.reload).to have_attributes(
+        expect(primary_tag_name_for(tag, 'en')).to have_attributes(
             language_code: 'en', script_code: 'Latn',
             primary_flg: true, tag_id: tag.id)
         expect(tag.category).to eq('character')
@@ -223,8 +235,10 @@ RSpec.describe Tag, type: :model do
             locale, 'same_name', category: :character)
 
         expect(english).not_to eq(japanese)
-        expect(english.tag_name.language_code).to eq('en')
-        expect(japanese.tag_name.language_code).to eq('ja')
+        expect(primary_tag_name_for(english, 'en')).to have_attributes(
+            name: 'same_name', tag_id: english.id, language_code: 'en', primary_flg: true)
+        expect(primary_tag_name_for(japanese, 'ja')).to have_attributes(
+            name: 'same_name', tag_id: japanese.id, language_code: 'ja', primary_flg: true)
         expect(described_class.find_or_create_by_tag_name!(
             locale, 'same_name', category: :general)).to eq(english)
         expect(described_class.find_or_create_by_tag_name!(
@@ -232,7 +246,7 @@ RSpec.describe Tag, type: :model do
       end
 
       it 'preserves the V1 representative when looking up another primary language' do
-        tag = create(:tag, name: '日本語代表名')
+        tag = create(:tag, primary_name: '日本語代表名')
         representative_id = tag.tag_name_id
         english = create(:tag_name, name: 'english_primary', tag:,
             language_code: 'en', script_code: 'Latn')
@@ -251,7 +265,7 @@ RSpec.describe Tag, type: :model do
             with_tagme: false, with_no_deerjikist: false)
 
         expect(tags.length).to eq(1)
-        expect(tags.first.tag_name).to have_attributes(
+        expect(primary_tag_name_for(tags.first, 'en')).to have_attributes(
             name: 'normalised_english', language_code: 'en', script_code: 'Latn',
             primary_flg: true, tag_id: tags.first.id)
       end
@@ -265,7 +279,7 @@ RSpec.describe Tag, type: :model do
             '  lookup_new  ', category: :character)
       }.to change(Tag, :count).by(1).and change(TagName, :count).by(1)
 
-      expect(tag.name).to eq('lookup_new')
+      expect(primary_tag_name_for(tag, 'ja').name).to eq('lookup_new')
       expect(tag.category).to eq('character')
     end
 
@@ -290,7 +304,7 @@ RSpec.describe Tag, type: :model do
 
     it 'reuses the owning tag through another alias' do
       tag = create(:tag)
-      primary = tag.tag_name
+      primary = primary_tag_name_for(tag, 'ja')
       alias_name = create(:tag_name, :alias, name: 'lookup_alias', tag:)
       other_alias = create(:tag_name, :alias, name: 'lookup_other_alias', tag:)
 
@@ -304,14 +318,14 @@ RSpec.describe Tag, type: :model do
         expect(found).to eq(tag)
       }.to change(Tag, :count).by(0).and change(TagName, :count).by(0)
 
-      expect(tag.reload.tag_name).to eq(primary)
+      expect(primary_tag_name_for(tag.reload, 'ja')).to eq(primary)
     end
   end
 
   describe '.merge_tags!' do
     let!(:target_tag) { create(:tag, category: :general) }
     let!(:source_tag) { create(:tag, category: :general) }
-    let!(:source_tag_name) { source_tag.tag_name }
+    let!(:source_tag_name) { primary_tag_name_for(source_tag, 'ja') }
 
     let!(:post_record) do
       Post.create!(url: 'https://example.com/posts/1', title: 'test post')
@@ -363,7 +377,7 @@ RSpec.describe Tag, type: :model do
 
     it 'keeps source history and records the new target alias after deleting the source' do
       user = create_member_user!
-      source_name = source_tag.name
+      source_name = primary_tag_name_for(source_tag, 'ja').name
       source_alias = create(:tag_name, :alias, name: 'merge_alias', tag: source_tag)
       TagVersioning.ensure_snapshot!(source_tag, created_by_user: user)
       original_version = source_tag.tag_versions.first
@@ -380,7 +394,7 @@ RSpec.describe Tag, type: :model do
 
       target_versions = target_tag.tag_versions.order(:version_no)
       expect(target_versions.pluck(:event_type)).to eq(['create', 'update'])
-      expect(target_versions.last.aliases.split).to eq([source_name])
+      expect(target_versions.last.aliases.split).to contain_exactly('merge_alias', source_name)
     end
 
     it 'deletes source relationships while preserving unrelated relationships' do
@@ -486,7 +500,7 @@ RSpec.describe Tag, type: :model do
 
       it 'rolls back earlier deletions, links, and history when a later source has a wiki' do
         earlier_source = create(:tag)
-        earlier_name = earlier_source.tag_name
+        earlier_name = primary_tag_name_for(earlier_source, 'ja')
         source_section = create(:post_tag_section, post: post_record,
                                                    tag: source_tag,
                                                    begin_ms: 1000, end_ms: 2000)
diff --git a/backend/spec/models/version_record_spec.rb b/backend/spec/models/version_record_spec.rb
index e76f4e7..32a28ed 100644
--- a/backend/spec/models/version_record_spec.rb
+++ b/backend/spec/models/version_record_spec.rb
@@ -1,7 +1,7 @@
 require 'rails_helper'
 
 RSpec.describe VersionRecord, type: :model do
-  let!(:tag) { create(:tag, name: 'version_record_tag') }
+  let!(:tag) { create(:tag, primary_name: 'version_record_tag') }
   let!(:nico_tag) { create(:external_tag, name: 'version_record_tag') }
 
   it 'makes TagVersion read only after create' do
@@ -9,7 +9,7 @@ RSpec.describe VersionRecord, type: :model do
       tag: tag,
       version_no: 1,
       event_type: 'create',
-      name: tag.name,
+      name: primary_tag_name_for(tag, 'ja').name,
       category: tag.category,
       aliases: '',
       parent_tag_ids: '',
@@ -27,7 +27,7 @@ RSpec.describe VersionRecord, type: :model do
       tag: tag,
       version_no: 1,
       event_type: 'create',
-      name: tag.name,
+      name: primary_tag_name_for(tag, 'ja').name,
       category: tag.category,
       aliases: '',
       parent_tag_ids: '',
diff --git a/backend/spec/requests/deerjikists_spec.rb b/backend/spec/requests/deerjikists_spec.rb
index 136fc2d..8d26658 100644
--- a/backend/spec/requests/deerjikists_spec.rb
+++ b/backend/spec/requests/deerjikists_spec.rb
@@ -30,7 +30,7 @@ RSpec.describe 'Deerjikists API', type: :request do
 
         expect(json['tag']).to be_a(Hash)
         expect(json['tag']['id']).to eq(tag1.id)
-        expect(json['tag']['name']).to eq(tag1.name)
+        expect(json['tag']['name']).to eq(primary_tag_name_for(tag1, 'ja').name)
       end
     end
 
diff --git a/backend/spec/requests/error_responses_spec.rb b/backend/spec/requests/error_responses_spec.rb
index a7b5191..81c3ab9 100644
--- a/backend/spec/requests/error_responses_spec.rb
+++ b/backend/spec/requests/error_responses_spec.rb
@@ -15,7 +15,7 @@ RSpec.describe 'error responses', type: :request do
 
     it 'returns a stable field-error payload for unprocessable requests' do
       member = create(:user, :member)
-      tag = create(:tag, :general, name: 'error_response_tag')
+      tag = create(:tag, category: :general, primary_name: 'error_response_tag')
       sign_in_as(member)
 
       patch "/tags/#{ tag.id }", params: { category: 'nico' }
diff --git a/backend/spec/requests/gekanator_learning_spec.rb b/backend/spec/requests/gekanator_learning_spec.rb
index 855bb75..7154683 100644
--- a/backend/spec/requests/gekanator_learning_spec.rb
+++ b/backend/spec/requests/gekanator_learning_spec.rb
@@ -898,22 +898,22 @@ RSpec.describe 'Gekanator learning API', type: :request do
 
   describe 'GET /gekanator/questions' do
     it 'omits questions for deprecated tags' do
-      active_tag = create(:tag, name: 'active_question_tag', category: :general)
+      active_tag = create(:tag, primary_name: 'active_question_tag', category: :general)
       deprecated_tag = create(:tag,
-          name: 'deprecated_question_tag',
+          primary_name: 'deprecated_question_tag',
           category: :general,
           deprecated_at: Time.current)
 
       [active_tag, deprecated_tag].each do |question_tag|
         GekanatorQuestion.create!(
-          text: "#{ question_tag.name }?",
+          text: "#{ primary_tag_name_for(question_tag, 'ja').name }?",
           kind: 'tag',
           source: 'admin_curated',
           status: 'accepted',
           priority_weight: 1.0,
           condition: {
             type: 'tag',
-            key: "#{ question_tag.category }:#{ question_tag.name }"
+            key: "#{ question_tag.category }:#{ primary_tag_name_for(question_tag, 'ja').name }"
           },
           created_by: admin
         )
diff --git a/backend/spec/requests/gekanator_posts_spec.rb b/backend/spec/requests/gekanator_posts_spec.rb
index 7fe627a..8cfa46f 100644
--- a/backend/spec/requests/gekanator_posts_spec.rb
+++ b/backend/spec/requests/gekanator_posts_spec.rb
@@ -4,9 +4,9 @@ require 'rails_helper'
 RSpec.describe 'Gekanator posts API', type: :request do
   describe 'GET /gekanator/posts' do
     it 'omits deprecated tags and returns the stored similarity cosine' do
-      active_tag = create(:tag, name: 'active tag', category: :general)
+      active_tag = create(:tag, primary_name: 'active tag', category: :general)
       deprecated_tag = create(:tag,
-          name: 'deprecated tag',
+          primary_name: 'deprecated tag',
           category: :general,
           deprecated_at: Time.current)
       post_record = Post.create!(title: 'source', url: 'https://example.com/source')
diff --git a/backend/spec/requests/locale_write_paths_spec.rb b/backend/spec/requests/locale_write_paths_spec.rb
index 7d269dc..6c0ef8f 100644
--- a/backend/spec/requests/locale_write_paths_spec.rb
+++ b/backend/spec/requests/locale_write_paths_spec.rb
@@ -96,7 +96,7 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
         tag = create(:tag)
 
         put "/tags/#{ tag.id }", params: locale_params.merge(
-            name: tag.name, category: tag.category, deprecated: false,
+            name: primary_tag_name_for(tag, 'ja').name, category: tag.category, deprecated: false,
             aliases: '', parent_tags: 'locale_parent')
 
         expect(response).to have_http_status(:ok), response.body
@@ -110,11 +110,11 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
     let!(:english_name) do
       create(:tag_name, name: 'shared_name', language_code: 'en', script_code: 'Latn')
     end
-    let!(:english_tag) { create(:tag, tag_name: english_name, category: :material) }
+    let!(:english_tag) { create(:tag, primary_tag_name: english_name, category: :material) }
     let!(:japanese_tag) do
-      create(:tag, name: 'temporary_japanese', category: :general).tap do |tag|
+      create(:tag, primary_name: 'temporary_japanese', category: :general).tap do |tag|
         # Isolate lookup from the separately tested uniqueness validation.
-        tag.tag_name.update_columns(name: 'shared_name')
+        primary_tag_name_for(tag, 'ja').update_columns(name: 'shared_name')
       end
     end
 
@@ -157,7 +157,8 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
       child = create(:tag)
 
       put "/tags/#{ child.id }", params: {
-          locale: 'en', name: child.name, category: child.category, deprecated: false,
+          locale: 'en', 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
@@ -180,23 +181,24 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
     let!(:english_name) do
       create(:tag_name, name: 'english_original', language_code: 'en', script_code: 'Latn')
     end
-    let!(:tag) { create(:tag, tag_name: english_name) }
+    let!(:tag) { create(:tag, primary_tag_name: english_name) }
 
     it 'allows a rename to a name already used in Japanese' do
-      japanese = create(:tag, name: 'rename_target')
+      japanese = create(:tag, primary_name: 'rename_target')
 
       patch "/tags/#{ tag.id }", params: { locale: 'en', name: 'rename_target' }
 
       expect(response).to have_http_status(:ok), response.body
-      expect(tag.reload.tag_name).to have_attributes(
+      expect(primary_tag_name_for(tag.reload, 'en')).to have_attributes(
           name: 'rename_target', language_code: 'en', tag_id: tag.id)
-      expect(japanese.reload.tag_name).to have_attributes(
+      expect(primary_tag_name_for(japanese.reload, 'ja')).to have_attributes(
           name: 'rename_target', language_code: 'ja', tag_id: japanese.id)
     end
 
     it 'creates aliases in the requested language' do
       put "/tags/#{ tag.id }", params: {
-          locale: 'en', name: tag.name, category: tag.category, deprecated: false,
+          locale: 'en', name: english_name.name,
+          category: tag.category, deprecated: false,
           aliases: 'english_alias', parent_tags: '' }
 
       expect(response).to have_http_status(:ok), response.body
@@ -210,7 +212,7 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
     end
 
     it 'does not take an alias from another language' do
-      japanese = create(:tag, name: 'japanese_owner')
+      japanese = create(:tag, primary_name: 'japanese_owner')
       japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                 tag: japanese)
       expect(japanese_alias.reload).to have_attributes(
@@ -218,7 +220,8 @@ RSpec.describe 'Locale propagation on write paths', type: :request do
           primary_flg: false, language_code: 'ja')
 
       put "/tags/#{ tag.id }", params: {
-          locale: 'en', name: tag.name, category: tag.category, deprecated: false,
+          locale: 'en', name: english_name.name,
+          category: tag.category, deprecated: false,
           aliases: 'shared_alias', parent_tags: '' }
 
       expect(response).to have_http_status(:ok), response.body
diff --git a/backend/spec/requests/materials_spec.rb b/backend/spec/requests/materials_spec.rb
index 17ff0a9..14b5d8a 100644
--- a/backend/spec/requests/materials_spec.rb
+++ b/backend/spec/requests/materials_spec.rb
@@ -26,10 +26,10 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'GET /materials' do
     let!(:tag_a) do
-      create(:tag, name: 'material_index_a', category: :material)
+      create(:tag, primary_name: 'material_index_a', category: :material)
     end
     let!(:tag_b) do
-      create(:tag, name: 'material_index_b', category: :material)
+      create(:tag, primary_name: 'material_index_b', category: :material)
     end
 
     let!(:material_a) do
@@ -116,20 +116,20 @@ RSpec.describe 'Materials API', type: :request do
 
     it 'filters by descendant tags and returns stable parent tag groups' do
       root =
-          create(:tag, name: 'material_scope_root',
+          create(:tag, primary_name: 'material_scope_root',
                       category: :material)
       child_b =
-          create(:tag, name: 'material_scope_b',
+          create(:tag, primary_name: 'material_scope_b',
                       category: :material)
       child_a =
-          create(:tag, name: 'material_scope_a',
+          create(:tag, primary_name: 'material_scope_a',
                       category: :material)
       deprecated =
-          create(:tag, name: 'material_scope_old',
+          create(:tag, primary_name: 'material_scope_old',
                       category: :material,
                       deprecated_at: Time.current)
       grandchild =
-          create(:tag, name: 'material_scope_grandchild',
+          create(:tag, primary_name: 'material_scope_grandchild',
                       category: :material)
       root_material =
           build_material(tag: root, user: member_user,
@@ -179,7 +179,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'GET /materials/:id' do
     let!(:tag) do
-      create(:tag, name: 'material_show', category: :material)
+      create(:tag, primary_name: 'material_show', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'show.png'))
@@ -269,7 +269,7 @@ RSpec.describe 'Materials API', type: :request do
         expect(response).to have_http_status(:created)
 
         material = Material.order(:id).last
-        expect(material.tag.name).to eq('material_create_new')
+        expect(primary_tag_name_for(material.tag, 'ja').name).to eq('material_create_new')
         expect(material.tag.category).to eq('material')
         expect(material.created_by_user).to eq(member_user)
         expect(material.updated_by_user).to eq(member_user)
@@ -297,7 +297,8 @@ RSpec.describe 'Materials API', type: :request do
 
         expect(response).to have_http_status(:created)
 
-        tag = Tag.joins(:tag_name).find_by!(tag_names: { name: 'material_create_versioned_tag' })
+        tag = TagName.find_by!(name: 'material_create_versioned_tag',
+                               language_code: 'ja', primary_flg: true).tag
         version = tag.tag_versions.order(:version_no).last
 
         expect(version.event_type).to eq('create')
@@ -324,7 +325,7 @@ RSpec.describe 'Materials API', type: :request do
 
       it 'returns 422 when the existing tag is not material/character' do
         general_tag_name = create(:tag_name, name: 'material_create_general_tag')
-        create(:tag, tag_name: general_tag_name, category: :general)
+        create(:tag, primary_tag_name: general_tag_name, category: :general)
 
         post '/materials', params: {
           tag: 'material_create_general_tag',
@@ -345,7 +346,7 @@ RSpec.describe 'Materials API', type: :request do
         expect(response).to have_http_status(:created)
 
         material = Material.order(:id).last
-        expect(material.tag.name).to eq('material_create_url_only')
+        expect(primary_tag_name_for(material.tag, 'ja').name).to eq('material_create_url_only')
         expect(material.url).to eq('https://example.com/material-source')
         expect(material.file.attached?).to be(false)
       end
@@ -384,7 +385,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'PUT /materials/:id' do
     let!(:tag) do
-      create(:tag, name: 'material_update_old', category: :material)
+      create(:tag, primary_name: 'material_update_old', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'old.png'))
@@ -463,7 +464,7 @@ RSpec.describe 'Materials API', type: :request do
         expect(response).to have_http_status(:ok)
 
         material.reload
-        expect(material.tag.name).to eq('material_update_new')
+        expect(primary_tag_name_for(material.tag, 'ja').name).to eq('material_update_new')
         expect(material.tag.category).to eq('material')
         expect(material.url).to eq('https://example.com/updated-source')
         expect(material.updated_by_user).to eq(member_user)
@@ -497,7 +498,8 @@ RSpec.describe 'Materials API', type: :request do
 
         expect(response).to have_http_status(:ok)
 
-        tag = Tag.joins(:tag_name).find_by!(tag_names: { name: 'material_update_versioned_tag' })
+        tag = TagName.find_by!(name: 'material_update_versioned_tag',
+                               language_code: 'ja', primary_flg: true).tag
         version = tag.tag_versions.order(:version_no).last
 
         expect(version.event_type).to eq('create')
@@ -508,7 +510,7 @@ RSpec.describe 'Materials API', type: :request do
 
       it 'backfills a create tag_version for an existing material tag without history' do
         existing_tag =
-            create(:tag, name: 'material_update_existing_no_history',
+            create(:tag, primary_name: 'material_update_existing_no_history',
                         category: :material)
 
         expect(existing_tag.tag_versions).to be_empty
@@ -531,7 +533,7 @@ RSpec.describe 'Materials API', type: :request do
 
       it 'backfills a create tag_version for an existing character tag without history' do
         existing_tag =
-            create(:tag, name: 'material_update_character_no_history',
+            create(:tag, primary_name: 'material_update_character_no_history',
                         category: :character)
 
         expect(existing_tag.tag_versions).to be_empty
@@ -563,7 +565,7 @@ RSpec.describe 'Materials API', type: :request do
         expect(response).to have_http_status(:ok)
 
         material.reload
-        expect(material.tag.name).to eq('material_update_remove_file')
+        expect(primary_tag_name_for(material.tag, 'ja').name).to eq('material_update_remove_file')
         expect(material.url).to eq('https://example.com/updated-source')
         expect(material.updated_by_user).to eq(member_user)
         expect(material.file.attached?).to be(false)
@@ -651,8 +653,8 @@ RSpec.describe 'Materials API', type: :request do
   end
 
   describe 'GET /materials/download.zip' do
-    let!(:tag_a) { create(:tag, name: 'zip_a', category: :material) }
-    let!(:tag_b) { create(:tag, name: 'zip_b', category: :material) }
+    let!(:tag_a) { create(:tag, primary_name: 'zip_a', category: :material) }
+    let!(:tag_b) { create(:tag, primary_name: 'zip_b', category: :material) }
     let!(:material_a) do
       build_material(tag: tag_a, user: member_user,
                      file: dummy_upload(filename: 'a.png', body: 'zip-a'))
@@ -701,7 +703,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'GET /materials/versions' do
     let!(:tag) do
-      create(:tag, name: 'material_history', category: :material)
+      create(:tag, primary_name: 'material_history', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'history.png'))
@@ -776,7 +778,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'DELETE /materials/:id' do
     let!(:tag) do
-      create(:tag, name: 'material_destroy', category: :material)
+      create(:tag, primary_name: 'material_destroy', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'destroy.png'))
diff --git a/backend/spec/requests/nico_tags_spec.rb b/backend/spec/requests/nico_tags_spec.rb
index f800afb..50fdd0d 100644
--- a/backend/spec/requests/nico_tags_spec.rb
+++ b/backend/spec/requests/nico_tags_spec.rb
@@ -44,10 +44,10 @@ RSpec.describe 'NicoTags', type: :request do
       unlinked.update!(name: 'search_unlinked')
       other = create(:external_tag)
       other.update!(name: 'other')
-      destination = create(:tag, :general)
-      destination.tag_name.update!(name: 'destination_search')
+      destination = create(:tag, category: :general)
+      primary_tag_name_for(destination, 'ja').update!(name: 'destination_search')
       NicoTagRelation.create!(nico_tag: linked, tag: destination)
-      NicoTagRelation.create!(nico_tag: other, tag: create(:tag, :general))
+      NicoTagRelation.create!(nico_tag: other, tag: create(:tag, category: :general))
 
       get '/tags/nico', params: {
         name: 'search_',
@@ -151,7 +151,7 @@ RSpec.describe 'NicoTags', type: :request do
 
     it '404 when only an internal tag exists for the target id' do
       sign_in_as(member)
-      non_nico = create(:tag, :general)
+      non_nico = create(:tag, category: :general)
       expect(ExternalTag.exists?(non_nico.id)).to be(false)
       patch "/tags/nico/#{non_nico.id}", params: { tags: 'a b' }
       expect(response).to have_http_status(:not_found)
@@ -163,17 +163,17 @@ RSpec.describe 'NicoTags', type: :request do
       nico_tag = create(:external_tag, name: 'nico_tags_spec_source')
 
       linked_a_name = create(:tag_name, name: 'nico_linked_a')
-      linked_a = create(:tag, tag_name: linked_a_name, category: :general)
+      linked_a = create(:tag, primary_tag_name: linked_a_name, category: :general)
 
       linked_b_name = create(:tag_name, name: 'nico_linked_b')
-      linked_b = create(:tag, tag_name: linked_b_name, category: :general)
+      linked_b = create(:tag, primary_tag_name: linked_b_name, category: :general)
 
       NicoTagVersionRecorder.record!(external_tag: nico_tag,
                                      event_type: :create, created_by_user: admin)
 
       expect {
         patch "/tags/nico/#{nico_tag.id}", params: {
-          tags: "  #{linked_a.name}\n#{linked_b.name}  "
+          tags: "  #{ linked_a_name.name }\n#{ linked_b_name.name }  "
         }
       }.to change(TagVersion, :count).by(2)
        .and change(NicoTagVersion, :count).by(1)
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index 580271a..38bc115 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -114,7 +114,7 @@ RSpec.describe 'Posts API', type: :request do
   end
 
   let!(:tag_name) { create(:tag_name, name: 'spec_tag') }
-  let!(:tag)      { create(:tag, tag_name: tag_name, category: :general) }
+  let!(:tag)      { create(:tag, primary_tag_name: tag_name, category: :general) }
 
   let!(:post_record) do
     Post.create!(title: 'spec post', url: 'https://example.com/spec').tap do |p|
@@ -126,9 +126,9 @@ RSpec.describe 'Posts API', type: :request do
     let!(:user) { create_member_user! }
 
     let!(:tag_name) { create(:tag_name, name: 'spec_tag') }
-    let!(:tag) { create(:tag, tag_name:, category: :general) }
+    let!(:tag) { create(:tag, primary_tag_name: tag_name, category: :general) }
     let!(:tag_name2) { create(:tag_name, name: 'unko') }
-    let!(:tag2) { create(:tag, tag_name: tag_name2, category: :deerjikist) }
+    let!(:tag2) { create(:tag, primary_tag_name: tag_name2, category: :deerjikist) }
     let!(:alias_tag_name) { create(:tag_name, :alias, name: 'manko', tag:) }
 
     let!(:hit_post) do
@@ -267,7 +267,7 @@ RSpec.describe 'Posts API', type: :request do
       end
 
       it 'keeps internal name searches independent of colliding external ids' do
-        get '/posts', params: { tags: tag.name }
+        get '/posts', params: { tags: primary_tag_name_for(tag, 'ja').name }
 
         expect(response).to have_http_status(:ok)
         expect(json.fetch('count')).to eq(3)
@@ -286,7 +286,7 @@ RSpec.describe 'Posts API', type: :request do
 
       [nil, 'all'].each do |match|
         it "intersects internal and external matches with match=#{ match || 'omitted' }" do
-          params = { tags: "#{ tag.name } nico:search_external" }
+          params = { tags: "#{ primary_tag_name_for(tag, 'ja').name } nico:search_external" }
           params[:match] = match if match
 
           get '/posts', params: params
@@ -307,7 +307,8 @@ RSpec.describe 'Posts API', type: :request do
       end
 
       it 'excludes external matches from an internal tag search' do
-        get '/posts', params: { tags: "#{ tag.name } not:nico:search_external" }
+        get '/posts', params: {
+            tags: "#{ primary_tag_name_for(tag, 'ja').name } not:nico:search_external" }
 
         expect(response).to have_http_status(:ok)
         expect(json.fetch('count')).to eq(2)
@@ -316,7 +317,9 @@ RSpec.describe 'Posts API', type: :request do
       end
 
       it 'unions an external match with a negated internal match' do
-        get '/posts', params: { tags: "nico:search_external not:#{ tag.name }", match: 'any' }
+        get '/posts', params: {
+            tags: "nico:search_external not:#{ primary_tag_name_for(tag, 'ja').name }",
+            match: 'any' }
 
         expect(response).to have_http_status(:ok)
         expect(json.fetch('count')).to eq(2)
@@ -333,7 +336,9 @@ RSpec.describe 'Posts API', type: :request do
       end
 
       it 'applies the same mixed name search to the existing random endpoint' do
-        get '/posts/random', params: { tags: "#{ tag.name } nico:search_external", match: 'all' }
+        get '/posts/random', params: {
+            tags: "#{ primary_tag_name_for(tag, 'ja').name } nico:search_external",
+            match: 'all' }
 
         expect(response).to have_http_status(:ok)
         expect(json.fetch('id')).to eq(both_post.id)
@@ -342,13 +347,13 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when tags contain not:' do
       let!(:foo_tag_name) { create(:tag_name, name: 'not_spec_foo') }
-      let!(:foo_tag) { create(:tag, tag_name: foo_tag_name, category: :general) }
+      let!(:foo_tag) { create(:tag, primary_tag_name: foo_tag_name, category: :general) }
 
       let!(:bar_tag_name) { create(:tag_name, name: 'not_spec_bar') }
-      let!(:bar_tag) { create(:tag, tag_name: bar_tag_name, category: :general) }
+      let!(:bar_tag) { create(:tag, primary_tag_name: bar_tag_name, category: :general) }
 
       let!(:baz_tag_name) { create(:tag_name, name: 'not_spec_baz') }
-      let!(:baz_tag) { create(:tag, tag_name: baz_tag_name, category: :general) }
+      let!(:baz_tag) { create(:tag, primary_tag_name: baz_tag_name, category: :general) }
 
       let!(:foo_alias_tag_name) do
         create(:tag_name, :alias, name: 'not_spec_foo_alias', tag: foo_tag)
@@ -739,7 +744,9 @@ RSpec.describe 'Posts API', type: :request do
           .find { _1['name'] == 'nico:post_index_external' }
 
       expect(post_json.fetch('tags')).to include(
-          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
+          a_hash_including('id' => tag.id,
+                           'name' => primary_tag_name_for(tag, 'ja').name,
+                           'category' => 'general'))
       expect(external_json).not_to be_nil
       expect_external_tag_json(external_json, external_tag)
     end
@@ -773,7 +780,7 @@ RSpec.describe 'Posts API', type: :request do
 
       it 'omits deprecated tags' do
         deprecated_tag = create(:tag,
-            name: 'deprecated_post_tag',
+            primary_name: 'deprecated_post_tag',
             category: :general,
             deprecated_at: Time.current)
         PostTag.create!(post: post_record, tag: deprecated_tag)
@@ -853,7 +860,7 @@ RSpec.describe 'Posts API', type: :request do
         tags =
             15.times.map do |i|
               tag_name = create(:tag_name, name: "show_query_tag_#{ i }")
-              tag = create(:tag, tag_name:, category: :general)
+              tag = create(:tag, primary_tag_name: tag_name, category: :general)
               create(:tag_name, :alias, name: "show_query_alias_#{ i }", tag:)
               PostTag.create!(post: post_record, tag:)
               tag
@@ -909,7 +916,9 @@ RSpec.describe 'Posts API', type: :request do
         expect(response).to have_http_status(:ok)
 
         expect(json.fetch('tags')).to include(
-            a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
+            a_hash_including('id' => tag.id,
+                             'name' => primary_tag_name_for(tag, 'ja').name,
+                             'category' => 'general'))
         external_json =
             json
             .fetch('tags')
@@ -1157,7 +1166,7 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'rejects a deprecated tag specified directly' do
       create(:tag,
-          name: 'deprecated_direct_tag',
+          primary_name: 'deprecated_direct_tag',
           category: :general,
           deprecated_at: Time.current)
       sign_in_as(member)
@@ -1176,16 +1185,16 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     it 'expands through multiple deprecated parent tags and saves active ancestors' do
-      child = create(:tag, name: 'active_child', category: :general)
+      child = create(:tag, primary_name: 'active_child', category: :general)
       deprecated_parent = create(:tag,
-          name: 'deprecated_parent',
+          primary_name: 'deprecated_parent',
           category: :general,
           deprecated_at: Time.current)
       deprecated_grandparent = create(:tag,
-          name: 'deprecated_grandparent',
+          primary_name: 'deprecated_grandparent',
           category: :general,
           deprecated_at: Time.current)
-      active_grandparent = create(:tag, name: 'active_grandparent', category: :general)
+      active_grandparent = create(:tag, primary_name: 'active_grandparent', category: :general)
       TagImplication.create!(tag: child, parent_tag: deprecated_parent)
       TagImplication.create!(tag: deprecated_parent, parent_tag: deprecated_grandparent)
       TagImplication.create!(tag: deprecated_grandparent, parent_tag: active_grandparent)
@@ -1199,7 +1208,8 @@ RSpec.describe 'Posts API', type: :request do
       )
 
       expect(response).to have_http_status(:created)
-      saved_names = Post.find(json.fetch('id')).tags.map(&:name)
+      saved_names = TagName.where(tag_id: Post.find(json.fetch('id')).tags.select(:id),
+                                  language_code: 'ja', primary_flg: true).pluck(:name)
       expect(saved_names).to include('active_child', 'active_grandparent')
       expect(saved_names).not_to include('deprecated_parent', 'deprecated_grandparent')
     end
@@ -1302,7 +1312,8 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:created)
 
       created_post = Post.find(json.fetch('id'))
-      tag = Tag.joins(:tag_name).find_by!(tag_names: { name: '伊地知ニジカ' })
+      tag = TagName.find_by!(name: '伊地知ニジカ',
+                             language_code: 'ja', primary_flg: true).tag
       section = PostTagSection.find_by!(post: created_post, tag:)
 
       expect(section.begin_ms).to eq(60_000)
@@ -1322,7 +1333,8 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:created)
 
       created_post = Post.find(json.fetch('id'))
-      tag = Tag.joins(:tag_name).find_by!(tag_names: { name: '伊地知ニジカ' })
+      tag = TagName.find_by!(name: '伊地知ニジカ',
+                             language_code: 'ja', primary_flg: true).tag
       expect(PostTagSection.find_by(post: created_post, tag:)).to be_nil
     end
 
@@ -1339,7 +1351,8 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:created)
 
       created_post = Post.find(json.fetch('id'))
-      tag = Tag.joins(:tag_name).find_by!(tag_names: { name: '伊地知ニジカ' })
+      tag = TagName.find_by!(name: '伊地知ニジカ',
+                             language_code: 'ja', primary_flg: true).tag
       expect(PostTagSection.find_by(post: created_post, tag:)).to be_nil
     end
 
@@ -1656,7 +1669,7 @@ RSpec.describe 'Posts API', type: :request do
                                 begin_ms: 1000, end_ms: 2000)
 
       tn2 = create(:tag_name, name: 'spec_tag_2')
-      replacement_tag = create(:tag, tag_name: tn2, category: :general)
+      replacement_tag = create(:tag, primary_tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1705,7 +1718,7 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'rejects a deprecated tag specified directly' do
       create(:tag,
-          name: 'deprecated_update_tag',
+          primary_name: 'deprecated_update_tag',
           category: :general,
           deprecated_at: Time.current)
       sign_in_as(member)
@@ -1962,7 +1975,7 @@ RSpec.describe 'Posts API', type: :request do
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
             title: 'updated by me',
-            tags: "spec_tag #{Tag.no_deerjikist.name}")
+            tags: "spec_tag #{primary_tag_name_for(Tag.no_deerjikist, 'ja').name}")
 
         expect(response).to have_http_status(:conflict)
 
@@ -1994,7 +2007,8 @@ RSpec.describe 'Posts API', type: :request do
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
             title: post_record.title,
-            tags: "spec_tag #{Tag.no_deerjikist.name} incoming_added_tag")
+            tags: ['spec_tag', primary_tag_name_for(Tag.no_deerjikist, 'ja').name,
+                   'incoming_added_tag'].join(' '))
 
         expect(response).to have_http_status(:conflict)
 
@@ -2024,15 +2038,17 @@ RSpec.describe 'Posts API', type: :request do
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
             title: post_record.title,
-            tags: "spec_tag #{Tag.no_deerjikist.name} incoming_merge_tag",
+            tags: ['spec_tag', primary_tag_name_for(Tag.no_deerjikist, 'ja').name,
+                   'incoming_merge_tag'].join(' '),
             merge: '1')
 
         expect(response).to have_http_status(:ok)
 
-        names = post_record.reload.tags.map(&:name)
+        names = TagName.where(tag_id: post_record.reload.tags.select(:id),
+                              language_code: 'ja', primary_flg: true).pluck(:name)
 
         expect(names).to include('spec_tag')
-        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).to include(primary_tag_name_for(Tag.no_deerjikist, 'ja').name)
         expect(names).to include('current_merge_tag')
         expect(names).to include('incoming_merge_tag')
       end
@@ -2055,14 +2071,15 @@ RSpec.describe 'Posts API', type: :request do
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
             title: post_record.title,
-            tags: "spec_tag #{ Tag.no_deerjikist.name }")
+            tags: "spec_tag #{ primary_tag_name_for(Tag.no_deerjikist, 'ja').name }")
 
         expect(response).to have_http_status(:ok)
 
-        names = post_record.reload.tags.map(&:name)
+        names = TagName.where(tag_id: post_record.reload.tags.select(:id),
+                              language_code: 'ja', primary_flg: true).pluck(:name)
 
         expect(names).to include('spec_tag')
-        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).to include(primary_tag_name_for(Tag.no_deerjikist, 'ja').name)
         expect(post_record.external_tags).to contain_exactly(nico_tag)
       end
 
@@ -2077,14 +2094,15 @@ RSpec.describe 'Posts API', type: :request do
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
             title: 'updated title',
-            tags: "spec_tag #{ Tag.no_deerjikist.name }")
+            tags: "spec_tag #{ primary_tag_name_for(Tag.no_deerjikist, 'ja').name }")
 
         expect(response).to have_http_status(:ok)
 
-        names = post_record.reload.tags.map(&:name)
+        names = TagName.where(tag_id: post_record.reload.tags.select(:id),
+                              language_code: 'ja', primary_flg: true).pluck(:name)
 
         expect(names).to include('spec_tag')
-        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).to include(primary_tag_name_for(Tag.no_deerjikist, 'ja').name)
         expect(post_record.external_tags).to contain_exactly(nico_tag)
 
         external_json =
@@ -2112,16 +2130,17 @@ RSpec.describe 'Posts API', type: :request do
         put "/posts/#{post_record.id}", params: post_write_params(
             base_version_no: base_version.version_no,
             title: post_record.title,
-            tags: "spec_tag #{ Tag.no_deerjikist.name }")
+            tags: "spec_tag #{ primary_tag_name_for(Tag.no_deerjikist, 'ja').name }")
 
         expect(response).to have_http_status(:ok)
 
-        names = post_record.reload.tags.map(&:name)
+        names = TagName.where(tag_id: post_record.reload.tags.select(:id),
+                              language_code: 'ja', primary_flg: true).pluck(:name)
 
         expect(post_record.external_tags).to contain_exactly(nico_tag)
         expect(names).to include('spec_tag')
-        expect(names).to include(Tag.no_deerjikist.name)
-        expect(names).not_to include(linked_tag.name)
+        expect(names).to include(primary_tag_name_for(Tag.no_deerjikist, 'ja').name)
+        expect(names).not_to include(primary_tag_name_for(linked_tag, 'ja').name)
       end
 
       it 'force-updates stale posts without base_version_no' do
@@ -2137,7 +2156,7 @@ RSpec.describe 'Posts API', type: :request do
 
         put "/posts/#{post_record.id}", params: post_write_params(
             title: 'forced title',
-            tags: "spec_tag #{Tag.no_deerjikist.name}",
+            tags: "spec_tag #{primary_tag_name_for(Tag.no_deerjikist, 'ja').name}",
             force: '1')
 
         expect(response).to have_http_status(:ok)
@@ -2191,7 +2210,7 @@ RSpec.describe 'Posts API', type: :request do
     let(:oc_before) { Time.zone.local(2020, 1, 1, 0, 0, 0) }
 
     let!(:tag_name2) { create(:tag_name, name: 'spec_tag_2') }
-    let!(:tag2) { create(:tag, tag_name: tag_name2, category: :general) }
+    let!(:tag2) { create(:tag, primary_tag_name: tag_name2, category: :general) }
 
     def snapshot_tags(post)
       post.snapshot_tag_names.join(' ')
@@ -2416,7 +2435,7 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:ok)
       expect(json.fetch('count')).to eq(3)
       expect(json.fetch('versions').first.fetch('tags')).to include(
-          'name' => tag2.name, 'type' => 'context')
+          'name' => primary_tag_name_for(tag2, 'ja').name, 'type' => 'context')
     end
 
     it 'filters versions by tag when the current snapshot includes the tag' do
@@ -2550,7 +2569,7 @@ RSpec.describe 'Posts API', type: :request do
       base_version = create_post_version_for!(post_record)
 
       tag_name2 = create(:tag_name, name: 'spec_tag_2')
-      create(:tag, tag_name: tag_name2, category: :general)
+      create(:tag, primary_tag_name: tag_name2, category: :general)
 
       expect do
         put "/posts/#{post_record.id}", params: post_write_params(
@@ -2715,7 +2734,7 @@ RSpec.describe 'Posts API', type: :request do
       base_version = create_post_version_for!(post_record.reload)
 
       tag_name2 = create(:tag_name, name: 'spec_tag_2')
-      tag2 = create(:tag, tag_name: tag_name2, category: :general)
+      tag2 = create(:tag, primary_tag_name: tag_name2, category: :general)
 
       expect {
         put "/posts/#{post_record.id}", params: post_write_params(
diff --git a/backend/spec/requests/tag_versions_spec.rb b/backend/spec/requests/tag_versions_spec.rb
index f9802a4..4aa441b 100644
--- a/backend/spec/requests/tag_versions_spec.rb
+++ b/backend/spec/requests/tag_versions_spec.rb
@@ -3,13 +3,13 @@ require 'rails_helper'
 RSpec.describe 'TagVersions API', type: :request do
   let(:member) { create(:user, :member, name: 'version member') }
 
-  let!(:tag) { create(:tag, name: 'tag_versions_target', category: :general) }
-  let!(:other_tag) { create(:tag, name: 'tag_versions_other', category: :general) }
+  let!(:tag) { create(:tag, primary_name: 'tag_versions_target', category: :general) }
+  let!(:other_tag) { create(:tag, primary_name: 'tag_versions_other', category: :general) }
 
-  let!(:parent_shared) { create(:tag, name: 'parent_shared', category: :general) }
-  let!(:parent_old) { create(:tag, name: 'parent_old', category: :general) }
-  let!(:parent_new) { create(:tag, name: 'parent_new', category: :general) }
-  let!(:other_parent) { create(:tag, name: 'other_parent', category: :general) }
+  let!(:parent_shared) { create(:tag, primary_name: 'parent_shared', category: :general) }
+  let!(:parent_old) { create(:tag, primary_name: 'parent_old', category: :general) }
+  let!(:parent_new) { create(:tag, primary_name: 'parent_new', category: :general) }
+  let!(:other_parent) { create(:tag, primary_name: 'other_parent', category: :general) }
 
   let(:t_v1) { Time.zone.local(2020, 1, 1, 12, 0, 0) }
   let(:t_v2) { Time.zone.local(2020, 1, 2, 12, 0, 0) }
@@ -211,7 +211,7 @@ RSpec.describe 'TagVersions API', type: :request do
     end
 
     it 'returns empty when the specified tag has no versions' do
-      fresh_tag = create(:tag, name: 'no_versions_tag', category: :general)
+      fresh_tag = create(:tag, primary_name: 'no_versions_tag', category: :general)
 
       get '/tags/versions', params: { id: fresh_tag.id }
 
@@ -233,7 +233,7 @@ RSpec.describe 'TagVersions API', type: :request do
 
     it 'does not create tag versions by wiki updates when tag has no versions yet' do
       wiki_tag_name = create(:tag_name, name: 'tag_versions_from_wiki')
-      wiki_tag = create(:tag, tag_name: wiki_tag_name, category: :general)
+      wiki_tag = create(:tag, primary_tag_name: wiki_tag_name, category: :general)
 
       wiki_page =
           Wiki::Commit.create_content!(
diff --git a/backend/spec/requests/tag_wiki_history_integrity_spec.rb b/backend/spec/requests/tag_wiki_history_integrity_spec.rb
index 33401e9..a464c05 100644
--- a/backend/spec/requests/tag_wiki_history_integrity_spec.rb
+++ b/backend/spec/requests/tag_wiki_history_integrity_spec.rb
@@ -9,12 +9,12 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
 
   def create_tag! name:, category: :general
     tag_name = create(:tag_name, name:)
-    create(:tag, tag_name:, category:)
+    create(:tag, primary_tag_name: tag_name, category:)
   end
 
   def create_wiki_for_tag! tag:, body: 'wiki body', user: member_user
     Wiki::Commit.create_content!(
-        tag_name: tag.tag_name,
+        tag_name: primary_tag_name_for(tag, 'ja'),
         body:,
         created_by_user: user,
         message: 'init')
@@ -43,7 +43,7 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
       wiki_page.reload
       version = wiki_page.wiki_versions.order(:version_no).last
 
-      expect(tag.name).to eq('patch_tag_wiki_after')
+      expect(primary_tag_name_for(tag, 'ja').name).to eq('patch_tag_wiki_after')
       expect(wiki_page.title).to eq('patch_tag_wiki_after')
 
       expect(version).to have_attributes(
@@ -72,7 +72,7 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
       tag.reload
       wiki_page.reload
 
-      expect(tag.name).to eq('patch_tag_category_only')
+      expect(primary_tag_name_for(tag, 'ja').name).to eq('patch_tag_category_only')
       expect(tag.category).to eq('meme')
       expect(wiki_page.wiki_versions.count).to eq(before_wiki_versions)
     end
@@ -101,7 +101,7 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
       wiki_page.reload
       version = wiki_page.wiki_versions.order(:version_no).last
 
-      expect(tag.name).to eq('put_tag_wiki_after')
+      expect(primary_tag_name_for(tag, 'ja').name).to eq('put_tag_wiki_after')
       expect(wiki_page.title).to eq('put_tag_wiki_after')
 
       expect(version).to have_attributes(
@@ -134,7 +134,7 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
       tag.reload
       wiki_page.reload
 
-      expect(tag.name).to eq('put_tag_category_only')
+      expect(primary_tag_name_for(tag, 'ja').name).to eq('put_tag_category_only')
       expect(tag.category).to eq('meme')
       expect(wiki_page.wiki_versions.count).to eq(before_wiki_versions)
     end
diff --git a/backend/spec/requests/tags_deerjikists_spec.rb b/backend/spec/requests/tags_deerjikists_spec.rb
index e0d3a3e..e0c2314 100644
--- a/backend/spec/requests/tags_deerjikists_spec.rb
+++ b/backend/spec/requests/tags_deerjikists_spec.rb
@@ -12,7 +12,7 @@ RSpec.describe 'Tags deerjikists API', type: :request do
   let(:guest) { create(:user, role: :guest) }
 
   before do
-    tag.tag_name.update!(name: 'deerjika')
+    primary_tag_name_for(tag, 'ja').update!(name: 'deerjika')
   end
 
   describe 'GET /tags/:id/deerjikists' do
@@ -344,7 +344,7 @@ RSpec.describe 'Tags deerjikists API', type: :request do
         end
 
         before do
-          other_tag.tag_name.update!(name: 'existing-deerjikist')
+          primary_tag_name_for(other_tag, 'ja').update!(name: 'existing-deerjikist')
         end
 
         it 'returns an indexed 422 error and rolls back the complete replacement' do
diff --git a/backend/spec/requests/tags_spec.rb b/backend/spec/requests/tags_spec.rb
index 8d10525..07478a0 100644
--- a/backend/spec/requests/tags_spec.rb
+++ b/backend/spec/requests/tags_spec.rb
@@ -3,12 +3,12 @@ require 'rails_helper'
 
 RSpec.describe 'Tags API', type: :request do
   let!(:tn) { create(:tag_name, name: 'spec_tag') }
-  let!(:tag) { create(:tag, tag_name: tn, category: :general) }
+  let!(:tag) { create(:tag, primary_tag_name: tn, category: :general) }
   let!(:alias_tn) { create(:tag_name, :alias, name: 'unko', tag:) }
   let!(:post) { Post.create!(url: 'https://example.com/unkounkounko') }
   let!(:post_tag) { PostTag.create!(post:, tag:) }
   let!(:tn2) { create(:tag_name, name: 'unknown') }
-  let!(:tag2) { create(:tag, tag_name: tn2, category: :general) }
+  let!(:tag2) { create(:tag, primary_tag_name: tn2, category: :general) }
 
   def response_tags
     json.fetch('tags')
@@ -38,7 +38,9 @@ RSpec.describe 'Tags API', type: :request do
       expect(response).to have_http_status(:ok)
       expect(json.fetch('count')).to eq(2)
       expect(response_tags).to contain_exactly(
-          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'),
+          a_hash_including('id' => tag.id,
+                           'name' => primary_tag_name_for(tag, 'ja').name,
+                           'category' => 'general'),
           a_hash_including(
               'id' => external.id,
               'name' => 'nico:spec_external',
@@ -91,7 +93,7 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by category' do
-      meme = create(:tag, name: 'meme_only', category: :meme)
+      meme = create(:tag, primary_name: 'meme_only', category: :meme)
 
       get '/tags', params: { category: 'meme' }
 
@@ -102,28 +104,28 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'filters tags by deprecated state' do
       deprecated_tag = create(:tag,
-          name: 'deprecated_filter',
+          primary_name: 'deprecated_filter',
           category: :general,
           deprecated_at: 1.day.from_now)
-      active_tag = create(:tag, name: 'active_filter', category: :general)
+      active_tag = create(:tag, primary_name: 'active_filter', category: :general)
 
       get '/tags', params: { name: '_filter', deprecated: '1' }
 
       expect(response).to have_http_status(:ok)
-      expect(response_names).to include(deprecated_tag.name)
-      expect(response_names).not_to include(active_tag.name)
+      expect(response_names).to include(primary_tag_name_for(deprecated_tag, 'ja').name)
+      expect(response_names).not_to include(primary_tag_name_for(active_tag, 'ja').name)
 
       get '/tags', params: { name: '_filter', deprecated: '0' }
 
       expect(response).to have_http_status(:ok)
-      expect(response_names).to include(active_tag.name)
-      expect(response_names).not_to include(deprecated_tag.name)
+      expect(response_names).to include(primary_tag_name_for(active_tag, 'ja').name)
+      expect(response_names).not_to include(primary_tag_name_for(deprecated_tag, 'ja').name)
     end
 
     it 'filters tags by post_count range' do
-      low = create(:tag, name: 'pc_low', category: :general)
-      mid = create(:tag, name: 'pc_mid', category: :general)
-      high = create(:tag, name: 'pc_high', category: :general)
+      low = create(:tag, primary_name: 'pc_low', category: :general)
+      mid = create(:tag, primary_name: 'pc_mid', category: :general)
+      high = create(:tag, primary_name: 'pc_high', category: :general)
 
       low.update_columns(post_count: 1)
       mid.update_columns(post_count: 3)
@@ -141,8 +143,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by created_at range' do
-      old_tag = create(:tag, name: 'created_old', category: :general)
-      new_tag = create(:tag, name: 'created_new', category: :general)
+      old_tag = create(:tag, primary_name: 'created_old', category: :general)
+      new_tag = create(:tag, primary_name: 'created_new', category: :general)
 
       old_time = Time.zone.local(2024, 1, 1, 0, 0, 0)
       new_time = Time.zone.local(2024, 2, 1, 0, 0, 0)
@@ -161,8 +163,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by updated_at range' do
-      old_tag = create(:tag, name: 'updated_old', category: :general)
-      new_tag = create(:tag, name: 'updated_new', category: :general)
+      old_tag = create(:tag, primary_name: 'updated_old', category: :general)
+      new_tag = create(:tag, primary_name: 'updated_new', category: :general)
 
       old_time = Time.zone.local(2024, 3, 1, 0, 0, 0)
       new_time = Time.zone.local(2024, 4, 1, 0, 0, 0)
@@ -181,12 +183,12 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'orders tags by custom category order' do
-      create(:tag, name: 'cat_deerjikist', category: :deerjikist)
-      create(:tag, name: 'cat_meme', category: :meme)
-      create(:tag, name: 'cat_character', category: :character)
-      create(:tag, name: 'cat_general', category: :general)
-      create(:tag, name: 'cat_material', category: :material)
-      create(:tag, name: 'cat_meta', category: :meta)
+      create(:tag, primary_name: 'cat_deerjikist', category: :deerjikist)
+      create(:tag, primary_name: 'cat_meme', category: :meme)
+      create(:tag, primary_name: 'cat_character', category: :character)
+      create(:tag, primary_name: 'cat_general', category: :general)
+      create(:tag, primary_name: 'cat_material', category: :material)
+      create(:tag, primary_name: 'cat_meta', category: :meta)
       create(:external_tag, name: 'cat_nico')
 
       get '/tags', params: { name: 'cat_', order: 'category:asc', limit: 20 }
@@ -201,19 +203,19 @@ RSpec.describe 'Tags API', type: :request do
     context 'with mixed legacy pagination' do
       let!(:first_tag) do
         create(:tag,
-            tag_name: create(:tag_name, name: 'a_mixed_page'),
+            primary_tag_name: create(:tag_name, name: 'a_mixed_page'),
             category: :meme)
       end
 
       let!(:middle_tag) do
         create(:tag,
-            tag_name: create(:tag_name, name: 'm_mixed_page'),
+            primary_tag_name: create(:tag_name, name: 'm_mixed_page'),
             category: :meta)
       end
 
       let!(:last_tag) do
         create(:tag,
-            tag_name: create(:tag_name, name: 'z_mixed_page'),
+            primary_tag_name: create(:tag_name, name: 'z_mixed_page'),
             category: :general)
       end
 
@@ -264,7 +266,7 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'paginates and keeps total count' do
       %w[pag_a pag_b pag_c].each do |name|
-        create(:tag, tag_name: create(:tag_name, name:), category: :general)
+        create(:tag, primary_tag_name: create(:tag_name, name:), category: :general)
       end
 
       get '/tags', params: { name: 'pag_', order: 'name:asc', page: 2, limit: 2 }
@@ -275,8 +277,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'falls back to default ordering when order is invalid' do
-      low = create(:tag, name: 'fallback_low', category: :general)
-      high = create(:tag, name: 'fallback_high', category: :general)
+      low = create(:tag, primary_name: 'fallback_low', category: :general)
+      high = create(:tag, primary_name: 'fallback_high', category: :general)
 
       low.update_columns(post_count: 1)
       high.update_columns(post_count: 9)
@@ -289,7 +291,7 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'normalises invalid page and limit' do
       %w[norm_a norm_b].each do |name|
-        create(:tag, tag_name: create(:tag_name, name:), category: :general)
+        create(:tag, primary_tag_name: create(:tag_name, name:), category: :general)
       end
 
       get '/tags', params: { name: 'norm_', order: 'name:asc', page: 0, limit: 0 }
@@ -302,7 +304,7 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'returns aliases and parent tags' do
       parent_tag = create(:tag,
-          tag_name: create(:tag_name, name: 'index_parent_tag'),
+          primary_tag_name: create(:tag_name, name: 'index_parent_tag'),
           category: :meme)
       TagImplication.create!(tag:, parent_tag:)
 
@@ -348,7 +350,7 @@ RSpec.describe 'Tags API', type: :request do
 
       it 'returns aliases and parent tags' do
         parent_tag = create(:tag,
-            tag_name: create(:tag_name, name: 'show_parent_tag'),
+            primary_tag_name: create(:tag_name, name: 'show_parent_tag'),
             category: :character)
         TagImplication.create!(tag:, parent_tag:)
 
@@ -395,7 +397,7 @@ RSpec.describe 'Tags API', type: :request do
     it 'prefers Tag when Tag and ExternalTag have the same id' do
       internal = create(
           :tag,
-          tag_name: create(:tag_name, name: 'internal_collision'))
+          primary_tag_name: create(:tag_name, name: 'internal_collision'))
       create(
           :external_tag,
           id: internal.id,
@@ -413,7 +415,7 @@ RSpec.describe 'Tags API', type: :request do
 
   describe 'GET /tags/autocomplete' do
     it 'combines internal and external matches without conflating equal ids' do
-      internal = create(:tag, category: :general, name: 'mixed_internal', post_count: 2)
+      internal = create(:tag, category: :general, primary_name: 'mixed_internal', post_count: 2)
       external = create(:external_tag, id: internal.id,
                                        name: 'mixed_external', post_count: 3)
 
@@ -452,7 +454,7 @@ RSpec.describe 'Tags API', type: :request do
     it 'limits the combined results to 20 and sorts ties by displayed name' do
       11.times do |i|
         name = "combined_#{ i.to_s.rjust(2, '0') }"
-        create(:tag, category: :general, name:, post_count: 1)
+        create(:tag, category: :general, primary_name: name, post_count: 1)
         create(:external_tag, name:, post_count: 1)
       end
 
@@ -464,8 +466,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'excludes external tags but preserves internal alias matches when nico is false' do
-      internal = create(:tag, category: :general, name: 'switch_internal', post_count: 1)
-      alias_target = create(:tag, category: :general, name: 'alias_target', post_count: 1)
+      internal = create(:tag, category: :general, primary_name: 'switch_internal', post_count: 1)
+      alias_target = create(:tag, category: :general, primary_name: 'alias_target', post_count: 1)
       create(:tag_name, :alias, name: 'switch_alias', tag: alias_target)
       create(:external_tag, name: 'switch_external', post_count: 1)
 
@@ -473,27 +475,30 @@ RSpec.describe 'Tags API', type: :request do
 
       expect(response).to have_http_status(:ok)
       expect(json).to contain_exactly(
-          a_hash_including('id' => internal.id, 'name' => internal.name),
-          a_hash_including('id' => alias_target.id, 'name' => alias_target.name,
+          a_hash_including('id' => internal.id,
+                           'name' => primary_tag_name_for(internal, 'ja').name),
+          a_hash_including('id' => alias_target.id,
+                           'name' => primary_tag_name_for(alias_target, 'ja').name,
                            'matched_alias' => 'switch_alias'))
     end
 
     ['%', '_'].each do |wildcard|
       it "treats #{ wildcard } literally for primary, alias, and external names" do
         literal = "literal#{ wildcard }match"
-        create(:tag, category: :general, name: literal, post_count: 1)
+        create(:tag, category: :general, primary_name: literal, post_count: 1)
         alias_target = create(
-            :tag, category: :general, name: 'literal_alias_target', post_count: 1)
+            :tag, category: :general, primary_name: 'literal_alias_target', post_count: 1)
         create(:tag_name, :alias, name: "#{ literal }_alias",
                                   tag: alias_target)
         create(:external_tag, name: literal, post_count: 1)
-        create(:tag, category: :general, name: 'literalXmatch', post_count: 2)
+        create(:tag, category: :general, primary_name: 'literalXmatch', post_count: 2)
         create(:external_tag, name: 'literalXmatch', post_count: 2)
 
         get '/tags/autocomplete', params: { q: "literal#{ wildcard }" }
 
         expect(json.map { |row| row.fetch('name') })
-            .to contain_exactly(literal, alias_target.name, "nico:#{ literal }")
+            .to contain_exactly(
+                literal, primary_tag_name_for(alias_target, 'ja').name, "nico:#{ literal }")
       end
     end
 
@@ -523,7 +528,7 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'omits deprecated tags' do
       deprecated_tag = create(:tag,
-          name: 'spec_deprecated',
+          primary_name: 'spec_deprecated',
           category: :general,
           deprecated_at: Time.current)
       deprecated_tag.update_columns(post_count: 1)
@@ -619,7 +624,7 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:ok)
 
         tag.reload
-        expect(tag.name).to eq('new')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('new')
         expect(tag.category).to eq('general')
 
         body = JSON.parse(response.body)
@@ -634,7 +639,7 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:ok)
 
         tag.reload
-        expect(tag.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('spec_tag')
         expect(tag.category).to eq('meme')
       end
 
@@ -644,7 +649,7 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:ok)
 
         tag.reload
-        expect(tag.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('spec_tag')
         expect(tag.category).to eq('general')
       end
 
@@ -654,7 +659,7 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:ok)
 
         tag.reload
-        expect(tag.name).to eq('n')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('n')
         expect(tag.category).to eq('meta')
       end
 
@@ -667,7 +672,7 @@ RSpec.describe 'Tags API', type: :request do
         patch "/tags/#{tag.id}", params: { name: 'new', category: 'nico' }
 
         expect(response).to have_http_status(:unprocessable_entity)
-        expect(tag.reload.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag.reload, 'ja').name).to eq('spec_tag')
         expect(tag.category).to eq('general')
       end
 
@@ -717,7 +722,7 @@ RSpec.describe 'Tags API', type: :request do
       it 'PATCH で tag の name を変更すると対応する wiki version を作成する' do
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'wiki body before',
                 created_by_user: member_user,
                 message: 'init')
@@ -745,7 +750,7 @@ RSpec.describe 'Tags API', type: :request do
       it 'tag の category だけを変更しても wiki version は作成しない' do
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'wiki body before',
                 created_by_user: member_user,
                 message: 'init')
@@ -781,8 +786,8 @@ RSpec.describe 'Tags API', type: :request do
 
         tag.reload
 
-        expect(tag.name).to eq('spec_tag')
-        expect(tag.tag_name).to have_attributes(
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag, 'ja')).to have_attributes(
             tag_id: tag.id, language_code: 'ja', primary_flg: true)
         expect(TagName.find_by!(language_code: 'ja', name: 'patch_roundtrip_target'))
             .to have_attributes(tag_id: tag.id, primary_flg: false)
@@ -791,7 +796,7 @@ RSpec.describe 'Tags API', type: :request do
       it '別 tag の正規名には変更できない' do
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'patch collision wiki',
                 created_by_user: member_user,
                 message: 'init')
@@ -803,10 +808,10 @@ RSpec.describe 'Tags API', type: :request do
           'name' => ['その名前は既に使はれてゐます．']
         )
 
-        expect(tag.reload.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag.reload, 'ja').name).to eq('spec_tag')
         expect(TagName.where(tag_id: tag.id, language_code: 'ja', primary_flg: false).pluck(:name))
             .to contain_exactly('unko')
-        expect(wiki_page.reload.tag_name).to eq(tag.tag_name)
+        expect(wiki_page.reload.tag_name).to eq(primary_tag_name_for(tag, 'ja'))
       end
 
       it 'system tag の name は変更できない' do
@@ -815,34 +820,34 @@ RSpec.describe 'Tags API', type: :request do
         patch "/tags/#{ system_tag.id }", params: { name: 'patch_system_tag_renamed' }
 
         expect(response).to have_http_status(:unprocessable_entity)
-        expect(system_tag.reload.name).to eq('bot操作')
+        expect(primary_tag_name_for(system_tag.reload, 'ja').name).to eq('bot操作')
       end
     end
   end
 
   describe 'GET /tags/with-depth' do
     let!(:root_meme) do
-      create(:tag, name: 'depth_a_root_meme', category: :meme)
+      create(:tag, primary_name: 'depth_a_root_meme', category: :meme)
     end
 
     let!(:root_material) do
-      create(:tag, name: 'depth_b_root_material', category: :material)
+      create(:tag, primary_name: 'depth_b_root_material', category: :material)
     end
 
     let!(:hidden_general_root) do
-      create(:tag, name: 'depth_hidden_general_root', category: :general)
+      create(:tag, primary_name: 'depth_hidden_general_root', category: :general)
     end
 
     let!(:child_character) do
-      create(:tag, name: 'depth_child_character', category: :character)
+      create(:tag, primary_name: 'depth_child_character', category: :character)
     end
 
     let!(:grandchild_material) do
-      create(:tag, name: 'depth_grandchild_material', category: :material)
+      create(:tag, primary_name: 'depth_grandchild_material', category: :material)
     end
 
     let!(:child_general) do
-      create(:tag, name: 'depth_child_general', category: :general)
+      create(:tag, primary_name: 'depth_child_general', category: :general)
     end
 
     before do
@@ -886,11 +891,11 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'passes through deprecated tags when finding children' do
       deprecated_middle = create(:tag,
-          name: 'depth_deprecated_middle',
+          primary_name: 'depth_deprecated_middle',
           category: :character,
           deprecated_at: Time.current)
       visible_descendant = create(:tag,
-          name: 'depth_visible_descendant',
+          primary_name: 'depth_visible_descendant',
           category: :material)
       TagImplication.create!(parent_tag: root_material, tag: deprecated_middle)
       TagImplication.create!(parent_tag: deprecated_middle, tag: visible_descendant)
@@ -904,18 +909,18 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'passes through multiple deprecated tags for roots and has_children' do
       active_child = create(:tag,
-          name: 'depth_active_child_below_deprecated',
+          primary_name: 'depth_active_child_below_deprecated',
           category: :character)
       deprecated_parent = create(:tag,
-          name: 'depth_deprecated_parent',
+          primary_name: 'depth_deprecated_parent',
           category: :character,
           deprecated_at: Time.current)
       deprecated_grandparent = create(:tag,
-          name: 'depth_deprecated_grandparent',
+          primary_name: 'depth_deprecated_grandparent',
           category: :material,
           deprecated_at: Time.current)
       active_ancestor = create(:tag,
-          name: 'depth_active_ancestor',
+          primary_name: 'depth_active_ancestor',
           category: :meme)
       TagImplication.create!(tag: active_child, parent_tag: deprecated_parent)
       TagImplication.create!(tag: deprecated_parent, parent_tag: deprecated_grandparent)
@@ -942,10 +947,10 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'treats an active tag with only deprecated ancestors as a root' do
       active_child = create(:tag,
-          name: 'depth_root_below_deprecated',
+          primary_name: 'depth_root_below_deprecated',
           category: :character)
       deprecated_parent = create(:tag,
-          name: 'depth_root_deprecated_parent',
+          primary_name: 'depth_root_deprecated_parent',
           category: :material,
           deprecated_at: Time.current)
       TagImplication.create!(tag: active_child, parent_tag: deprecated_parent)
@@ -962,11 +967,11 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'terminates when deprecated implications contain a cycle' do
       first = create(:tag,
-          name: 'depth_cycle_first',
+          primary_name: 'depth_cycle_first',
           category: :character,
           deprecated_at: Time.current)
       second = create(:tag,
-          name: 'depth_cycle_second',
+          primary_name: 'depth_cycle_second',
           category: :material,
           deprecated_at: Time.current)
       TagImplication.create!(tag: first, parent_tag: root_material)
@@ -994,19 +999,19 @@ RSpec.describe 'Tags API', type: :request do
     let!(:material_user) { create_member_user! }
 
     let!(:root_tag) do
-      create(:tag, name: 'materials_root', category: :material)
+      create(:tag, primary_name: 'materials_root', category: :material)
     end
 
     let!(:child_a_tag) do
-      create(:tag, name: 'materials_child_a', category: :material)
+      create(:tag, primary_name: 'materials_child_a', category: :material)
     end
 
     let!(:child_b_tag) do
-      create(:tag, name: 'materials_child_b', category: :character)
+      create(:tag, primary_name: 'materials_child_b', category: :character)
     end
 
     let!(:grandchild_tag) do
-      create(:tag, name: 'materials_grandchild', category: :material)
+      create(:tag, primary_name: 'materials_grandchild', category: :material)
     end
 
     let!(:root_material) do
@@ -1028,7 +1033,7 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'returns a tag tree with nested materials sorted by child name' do
-      get "/tags/name/#{ CGI.escape(root_tag.name) }/materials"
+      get "/tags/name/#{ CGI.escape(primary_tag_name_for(root_tag, 'ja').name) }/materials"
 
       expect(response).to have_http_status(:ok)
 
@@ -1119,7 +1124,7 @@ RSpec.describe 'Tags API', type: :request do
         }
 
         expect(response).to have_http_status(:unprocessable_entity)
-        expect(tag.reload.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag.reload, 'ja').name).to eq('spec_tag')
       end
 
       it 'category が空なら 422 を返す' do
@@ -1131,7 +1136,7 @@ RSpec.describe 'Tags API', type: :request do
         }
 
         expect(response).to have_http_status(:unprocessable_entity)
-        expect(tag.reload.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag.reload, 'ja').name).to eq('spec_tag')
         expect(tag.category).to eq('general')
       end
 
@@ -1151,10 +1156,10 @@ RSpec.describe 'Tags API', type: :request do
 
       it 'name, category, aliases, parent tags をまとめて更新できる' do
         old_parent = create(:tag,
-            tag_name: create(:tag_name, name: 'put_old_parent'),
+            primary_tag_name: create(:tag_name, name: 'put_old_parent'),
             category: :general)
         kept_parent = create(:tag,
-            tag_name: create(:tag_name, name: 'put_kept_parent'),
+            primary_tag_name: create(:tag_name, name: 'put_kept_parent'),
             category: :general)
         TagImplication.create!(tag:, parent_tag: old_parent)
         TagImplication.create!(tag:, parent_tag: kept_parent)
@@ -1171,7 +1176,7 @@ RSpec.describe 'Tags API', type: :request do
 
         tag.reload
 
-        expect(tag.name).to eq('put_renamed_tag')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('put_renamed_tag')
         expect(tag.category).to eq('meme')
 
         expect(TagName.find_by!(language_code: 'ja', name: 'put_alias_a'))
@@ -1179,17 +1184,16 @@ RSpec.describe 'Tags API', type: :request do
         expect(TagName.find_by!(language_code: 'ja', name: 'put_alias_b'))
             .to have_attributes(tag_id: tag.id, primary_flg: false)
 
-        old_name_alias = TagName.find_by(name: 'spec_tag')
+        old_name_alias = TagName.find_by(language_code: 'ja', name: 'spec_tag')
         expect(old_name_alias).to be_present
         expect(old_name_alias).to have_attributes(
             tag_id: tag.id, language_code: 'ja', primary_flg: false)
 
         expect(alias_tn.reload.tag_id).to be_nil
 
-        expect(tag.parents.map(&:name)).to contain_exactly(
-          'put_kept_parent',
-          'put_new_parent'
-        )
+        expect(TagName.where(tag_id: tag.parents.select(:id),
+                             language_code: 'ja', primary_flg: true).pluck(:name))
+            .to contain_exactly('put_kept_parent', 'put_new_parent')
 
         expect(TagImplication.where(tag:, parent_tag: old_parent)).not_to exist
 
@@ -1290,7 +1294,7 @@ RSpec.describe 'Tags API', type: :request do
 
         tag.reload
 
-        expect(tag.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('spec_tag')
         expect(TagName.find_by!(language_code: 'ja', name: 'put_roundtrip_b'))
             .to have_attributes(tag_id: tag.id, primary_flg: false)
         expect(TagName.where(tag_id: tag.id, language_code: 'ja', primary_flg: false).pluck(:name))
@@ -1309,10 +1313,10 @@ RSpec.describe 'Tags API', type: :request do
 
       it 'parent tag の snapshot も作成する' do
         old_parent = create(:tag,
-            tag_name: create(:tag_name, name: 'put_snapshot_old_parent'),
+            primary_tag_name: create(:tag_name, name: 'put_snapshot_old_parent'),
             category: :general)
         new_parent = create(:tag,
-            tag_name: create(:tag_name, name: 'put_snapshot_new_parent'),
+            primary_tag_name: create(:tag_name, name: 'put_snapshot_new_parent'),
             category: :general)
         TagImplication.create!(tag:, parent_tag: old_parent)
 
@@ -1320,7 +1324,7 @@ RSpec.describe 'Tags API', type: :request do
           name: 'spec_tag',
           category: 'general',
           aliases: 'unko',
-          parent_tags: new_parent.name,
+          parent_tags: primary_tag_name_for(new_parent, 'ja').name,
           deprecated: '0',
         }
 
@@ -1343,13 +1347,13 @@ RSpec.describe 'Tags API', type: :request do
 
         expect(response).to have_http_status(:unprocessable_entity)
 
-        expect(tag.reload.name).to eq('spec_tag')
+        expect(primary_tag_name_for(tag.reload, 'ja').name).to eq('spec_tag')
         expect(tag.category).to eq('general')
       end
 
       it 'system tag の name は変更できない' do
         system_tag = Tag.tagme
-        old_name = system_tag.name
+        old_name = primary_tag_name_for(system_tag, 'ja').name
         old_category = system_tag.category
 
         expect {
@@ -1364,14 +1368,14 @@ RSpec.describe 'Tags API', type: :request do
 
         expect(response).to have_http_status(:unprocessable_entity)
 
-        expect(system_tag.reload.name).to eq(old_name)
+        expect(primary_tag_name_for(system_tag.reload, 'ja').name).to eq(old_name)
         expect(system_tag.category).to eq(old_category)
       end
 
       it 'wiki を持つ tag を更新すると wiki version も作成する' do
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'wiki body before',
                 created_by_user: member_user,
                 message: 'init')
@@ -1410,7 +1414,7 @@ RSpec.describe 'Tags API', type: :request do
       it 'wiki を持つ tag を旧 alias へ戻しても wiki を新しい正本名へ移す' do
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'wiki body before',
                 created_by_user: member_user,
                 message: 'init')
@@ -1439,8 +1443,9 @@ RSpec.describe 'Tags API', type: :request do
 
         tag.reload
 
-        expect(wiki_page.reload.tag_name).to eq(tag.tag_name)
-        expect(TagName.find_by!(name: 'put_wiki_roundtrip_b').wiki_page).to be_nil
+        expect(wiki_page.reload.tag_name).to eq(primary_tag_name_for(tag, 'ja'))
+        expect(TagName.find_by!(language_code: 'ja', name: 'put_wiki_roundtrip_b').wiki_page)
+            .to be_nil
         expect(TagName.find_by!(language_code: 'ja', name: 'put_wiki_roundtrip_b'))
             .to have_attributes(tag_id: tag.id, primary_flg: false)
 
@@ -1452,7 +1457,7 @@ RSpec.describe 'Tags API', type: :request do
 
       it '別名を他 tag から奪った場合、奪はれた側の tag version も作成する' do
         old_owner = create(:tag,
-            tag_name: create(:tag_name, name: 'put_alias_old_owner'),
+            primary_tag_name: create(:tag_name, name: 'put_alias_old_owner'),
             category: :general)
         stolen_alias = create(:tag_name, :alias,
             name: 'put_stolen_alias',
@@ -1493,14 +1498,14 @@ RSpec.describe 'Tags API', type: :request do
 
       it '別 tag の alias 名を rename で奪へる' do
         old_owner = create(:tag,
-            tag_name: create(:tag_name, name: 'put_alias_collision_owner'),
+            primary_tag_name: create(:tag_name, name: 'put_alias_collision_owner'),
             category: :general)
         stolen_alias = create(:tag_name, :alias,
             name: 'put_alias_collision_name',
             tag: old_owner)
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'put collision wiki',
                 created_by_user: member_user,
                 message: 'init')
@@ -1519,7 +1524,7 @@ RSpec.describe 'Tags API', type: :request do
         old_owner.reload
         stolen_alias.reload
 
-        expect(tag.name).to eq('put_alias_collision_name')
+        expect(primary_tag_name_for(tag, 'ja').name).to eq('put_alias_collision_name')
         expect(stolen_alias).to have_attributes(
             tag_id: tag.id, language_code: 'ja', primary_flg: true)
         expect(TagName.find_by!(language_code: 'ja', name: 'spec_tag'))
@@ -1532,12 +1537,12 @@ RSpec.describe 'Tags API', type: :request do
 
         expect(old_owner_versions.last.event_type).to eq('update')
         expect(old_owner_versions.last.aliases.split).not_to include('put_alias_collision_name')
-        expect(wiki_page.reload.tag_name).to eq(tag.tag_name)
+        expect(wiki_page.reload.tag_name).to eq(primary_tag_name_for(tag, 'ja'))
       end
 
       it 'parent_tags に指定すると循環する tag は 422 にする' do
         child = create(:tag,
-            tag_name: create(:tag_name, name: 'put_cycle_child'),
+            primary_tag_name: create(:tag_name, name: 'put_cycle_child'),
             category: :general)
 
         TagImplication.create!(tag: child, parent_tag: tag)
@@ -1546,7 +1551,7 @@ RSpec.describe 'Tags API', type: :request do
           name: 'spec_tag',
           category: 'general',
           aliases: 'unko',
-          parent_tags: child.name,
+          parent_tags: primary_tag_name_for(child, 'ja').name,
           deprecated: '0',
         }
 
@@ -1558,7 +1563,7 @@ RSpec.describe 'Tags API', type: :request do
       it 'tag の name を変更すると対応する wiki version を作成する' do
         wiki_page =
             Wiki::Commit.create_content!(
-                tag_name: tag.tag_name,
+                tag_name: primary_tag_name_for(tag, 'ja'),
                 body: 'wiki body before',
                 created_by_user: member_user,
                 message: 'init')
diff --git a/backend/spec/requests/theatres_spec.rb b/backend/spec/requests/theatres_spec.rb
index 7fd3a0a..60fb964 100644
--- a/backend/spec/requests/theatres_spec.rb
+++ b/backend/spec/requests/theatres_spec.rb
@@ -396,7 +396,7 @@ RSpec.describe 'Theatres API', type: :request do
     end
 
     it 'finalizes skip when votes reach majority and stores voters and tag snapshots' do
-      tag = create(:tag, name: 'skip-target')
+      tag = create(:tag, primary_name: 'skip-target')
       PostTag.create!(post: niconico_post, tag:)
 
       TheatreSkipVote.create!(theatre:, post: niconico_post, user: member)
@@ -503,7 +503,7 @@ RSpec.describe 'Theatres API', type: :request do
     end
 
     it 'returns tag penalties and candidate weights for the current watchers' do
-      tag = create(:tag, name: 'heavy-tag')
+      tag = create(:tag, primary_name: 'heavy-tag')
       PostTag.create!(post: second_niconico_post, tag:)
       event = TheatreSkipEvent.create!(
         theatre:,
diff --git a/backend/spec/requests/wiki_spec.rb b/backend/spec/requests/wiki_spec.rb
index 7bf7c49..e1d31b1 100644
--- a/backend/spec/requests/wiki_spec.rb
+++ b/backend/spec/requests/wiki_spec.rb
@@ -20,7 +20,7 @@ RSpec.describe 'Wiki API', type: :request do
   end
   let!(:tag) do
     create(:tag,
-        tag_name: tn,
+        primary_tag_name: tn,
         category: :general,
         deprecated_at: Time.zone.local(2026, 6, 1))
   end
@@ -275,7 +275,7 @@ RSpec.describe 'Wiki API', type: :request do
 
       it 'wiki body だけを変更しても tag version は作成しない' do
         linked_tag_name = create(:tag_name, name: 'wiki_body_only_tag')
-        linked_tag = create(:tag, tag_name: linked_tag_name, category: :general)
+        linked_tag = create(:tag, primary_tag_name: linked_tag_name, category: :general)
 
         TagVersionRecorder.record!(
             tag: linked_tag,
@@ -535,7 +535,7 @@ RSpec.describe 'Wiki API', type: :request do
 
   it 'wiki title を変更すると対応する tag の version を作成する' do
     linked_tag_name = create(:tag_name, name: 'wiki_linked_tag_for_version')
-    linked_tag = create(:tag, tag_name: linked_tag_name, category: :general)
+    linked_tag = create(:tag, primary_tag_name: linked_tag_name, category: :general)
 
     linked_page =
         Wiki::Commit.create_content!(
@@ -563,7 +563,8 @@ RSpec.describe 'Wiki API', type: :request do
     expect(response).to have_http_status(:ok)
 
     linked_tag.reload
-    expect(linked_tag.name).to eq('wiki_linked_tag_for_version_renamed')
+    expect(primary_tag_name_for(linked_tag, 'ja').name)
+        .to eq('wiki_linked_tag_for_version_renamed')
 
     versions = linked_tag.tag_versions.order(:version_no)
 
diff --git a/backend/spec/services/material_sync_importer_spec.rb b/backend/spec/services/material_sync_importer_spec.rb
index 7d34dc6..b2734e3 100644
--- a/backend/spec/services/material_sync_importer_spec.rb
+++ b/backend/spec/services/material_sync_importer_spec.rb
@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe MaterialSyncImporter do
   let(:user) { create(:user, :member) }
-  let(:tag) { create(:tag, name: 'sync_tag', category: :material) }
+  let(:tag) { create(:tag, primary_name: 'sync_tag', category: :material) }
 
   def tempfile_for body
     Tempfile.new(['material-sync-importer', '.png']).tap do |file|
diff --git a/backend/spec/services/nico_tag_version_recorder_spec.rb b/backend/spec/services/nico_tag_version_recorder_spec.rb
index b7811c4..e48745b 100644
--- a/backend/spec/services/nico_tag_version_recorder_spec.rb
+++ b/backend/spec/services/nico_tag_version_recorder_spec.rb
@@ -65,7 +65,7 @@ RSpec.describe NicoTagVersionRecorder do
   end
 
   it 'records sorted linked internal names and later link removal' do
-    tags = ['z_link', 'a_link'].map { |name| create(:tag, name:, category: :general) }
+    tags = ['z_link', 'a_link'].map { |name| create(:tag, primary_name: name, category: :general) }
     tags.each { |tag| NicoTagRelation.create!(nico_tag: external_tag, tag:) }
 
     expect(record(:create).linked_tags).to eq('a_link z_link')
diff --git a/backend/spec/services/post_create_plan_spec.rb b/backend/spec/services/post_create_plan_spec.rb
index 209def7..5ff805a 100644
--- a/backend/spec/services/post_create_plan_spec.rb
+++ b/backend/spec/services/post_create_plan_spec.rb
@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe PostCreatePlan do
   def create_tag! name, category
-    create(:tag, name:, category:)
+    create(:tag, primary_name: name, category:)
   end
 
   before do
@@ -16,12 +16,15 @@ RSpec.describe PostCreatePlan do
     it 'resolves the matching language identity and its parents' do
       english_name = create(:tag_name, name: 'shared_name',
                                       language_code: 'en', script_code: 'Latn')
-      english = create(:tag, tag_name: english_name, category: :character)
-      japanese = create(:tag, name: 'temporary_japanese', category: :general)
+      english = create(:tag, primary_tag_name: english_name, category: :character)
+      japanese = create(:tag, primary_name: 'temporary_japanese', category: :general)
       # The DB permits this identity; validation has its own contract spec.
-      japanese.tag_name.update_columns(name: 'shared_name')
-      english_parent = create(:tag, name: 'english_parent', category: :material)
-      japanese_parent = create(:tag, name: 'japanese_parent', category: :general)
+      primary_tag_name_for(japanese, '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,
+                                    category: :material)
+      japanese_parent = create(:tag, primary_name: 'japanese_parent', category: :general)
       TagImplication.create!(tag: english, parent_tag: english_parent)
       TagImplication.create!(tag: japanese, parent_tag: japanese_parent)
 
@@ -30,15 +33,16 @@ RSpec.describe PostCreatePlan do
       expect(plan[:direct_tag_specs]).to eq(
           [{ name: 'shared_name', category: :character }])
       expect(plan[:post_tag_specs]).to include(
-          { name: english_parent.name, category: :material })
-      expect(plan[:post_tag_specs].pluck(:name)).not_to include(japanese_parent.name)
+          { name: primary_tag_name_for(english_parent, 'en').name, category: :material })
+      expect(plan[:post_tag_specs].pluck(:name))
+          .not_to include(primary_tag_name_for(japanese_parent, 'ja').name)
     end
 
     it 'resolves aliases within the requested language' do
-      japanese = create(:tag, name: 'japanese_canonical')
+      japanese = create(:tag, primary_name: 'japanese_canonical')
       english_name = create(:tag_name, name: 'english_canonical',
                                       language_code: 'en', script_code: 'Latn')
-      english = create(:tag, tag_name: english_name, category: :character)
+      english = create(:tag, primary_tag_name: english_name, category: :character)
       japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
                                                 tag: japanese)
       english_alias = create(:tag_name, :alias, name: 'shared_alias',
@@ -55,7 +59,7 @@ RSpec.describe PostCreatePlan do
       plan = described_class.new(attributes: { tags: 'shared_alias' }).build!(locale)
 
       expect(plan[:direct_tag_specs]).to eq(
-          [{ name: english.name, category: :character }])
+          [{ name: english_name.name, category: :character }])
     end
   end
 
diff --git a/backend/spec/services/post_import_previewer_spec.rb b/backend/spec/services/post_import_previewer_spec.rb
index 5bc7bcc..cb6888b 100644
--- a/backend/spec/services/post_import_previewer_spec.rb
+++ b/backend/spec/services/post_import_previewer_spec.rb
@@ -64,7 +64,7 @@ RSpec.describe PostImportPreviewer do
     end
 
     it 'applies metadata to automatic fields and recognises metadata tags' do
-      create(:tag, name: 'known-tag', category: :general)
+      create(:tag, primary_name: 'known-tag', category: :general)
       allow(PostMetadataFetcher).to receive(:fetch).and_return(
         title: 'metadata title',
         thumbnail_base: 'https://example.com/thumb.jpg',
diff --git a/backend/spec/services/wiki/commit_integrity_spec.rb b/backend/spec/services/wiki/commit_integrity_spec.rb
index 9351d75..42c42b7 100644
--- a/backend/spec/services/wiki/commit_integrity_spec.rb
+++ b/backend/spec/services/wiki/commit_integrity_spec.rb
@@ -77,7 +77,7 @@ RSpec.describe Wiki::Commit do
 
     it 'does not record tag_version on body-only wiki update' do
       tag_name = create(:tag_name, name: 'commit_integrity_linked_tag')
-      tag = create(:tag, tag_name:, category: :general)
+      tag = create(:tag, primary_tag_name: tag_name, category: :general)
 
       page =
           described_class.create_content!(
diff --git a/backend/spec/services/wiki/commit_spec.rb b/backend/spec/services/wiki/commit_spec.rb
index 5d82b34..122ac58 100644
--- a/backend/spec/services/wiki/commit_spec.rb
+++ b/backend/spec/services/wiki/commit_spec.rb
@@ -81,7 +81,7 @@ RSpec.describe Wiki::Commit do
 
     it 'does not record tag version when corresponding tag has no versions' do
       tag_name = create(:tag_name, name: 'commit_linked_tag_without_versions')
-      tag = create(:tag, tag_name:, category: :general)
+      tag = create(:tag, primary_tag_name: tag_name, category: :general)
 
       page =
           described_class.create_content!(
@@ -108,7 +108,7 @@ RSpec.describe Wiki::Commit do
 
     it 'does not record tag version when corresponding tag has no versions' do
       tag_name = create(:tag_name, name: 'commit_linked_tag_without_versions')
-      tag = create(:tag, tag_name:, category: :general)
+      tag = create(:tag, primary_tag_name: tag_name, category: :general)
 
       page =
           described_class.create_content!(
diff --git a/backend/spec/support/test_records.rb b/backend/spec/support/test_records.rb
index b6a7cd5..a814e68 100644
--- a/backend/spec/support/test_records.rb
+++ b/backend/spec/support/test_records.rb
@@ -1,4 +1,8 @@
 module TestRecords
+  def primary_tag_name_for(tag, language_code)
+    TagName.find_by!(tag_id: tag.id, language_code:, primary_flg: true)
+  end
+
   def create_member_user!
     User.create!(name: 'spec user',
                  inheritance_code: SecureRandom.hex(16),
diff --git a/backend/spec/tasks/nico_sync_spec.rb b/backend/spec/tasks/nico_sync_spec.rb
index e62a8f3..6ae8afa 100644
--- a/backend/spec/tasks/nico_sync_spec.rb
+++ b/backend/spec/tasks/nico_sync_spec.rb
@@ -64,7 +64,8 @@ RSpec.describe 'nico:sync' do
     run_rake_task('nico:sync')
 
     post.reload
-    active_tag_names = post.tags.joins(:tag_name).pluck('tag_names.name')
+    active_tag_names = TagName.where(tag_id: post.tags.select(:id),
+                                     language_code: 'ja', primary_flg: true).pluck(:name)
 
     expect(active_tag_names).to include('spec_kept')
     expect(post.external_tags).to contain_exactly(nico)
@@ -434,7 +435,9 @@ RSpec.describe 'nico:sync' do
 
     external = post.external_tags.sole
     expect(external.name).to eq('raw tag[]')
-    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(TagName.where(tag_id: post.tags.select(:id),
+                         language_code: 'ja', primary_flg: true).pluck(:name))
+        .not_to include('bot操作')
     expect(post.post_versions.order(:version_no).last.tags_json)
         .to include('external_tag_id' => external.id)
 
@@ -446,7 +449,9 @@ RSpec.describe 'nico:sync' do
       run_nico_sync_with_tags!([])
     }.to change(PostVersion, :count).by(1)
     expect(post.reload.external_tags).to be_empty
-    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(TagName.where(tag_id: post.tags.select(:id),
+                         language_code: 'ja', primary_flg: true).pluck(:name))
+        .not_to include('bot操作')
   end
 
   it 'サニタイズ後の既存外部タグを再利用し、その連携タグを記載する' do
@@ -563,7 +568,9 @@ RSpec.describe 'nico:sync' do
         name: 'AAA')
     expect(ExternalTag.exists?(platform: :nico, name: 'AAA?')).to be(false)
 
-    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(TagName.where(tag_id: post.tags.select(:id),
+                         language_code: 'ja', primary_flg: true).pluck(:name))
+        .not_to include('bot操作')
     expect(post.post_versions.order(:version_no).last.tags_json)
         .to include('external_tag_id' => external.id)
 
@@ -577,7 +584,9 @@ RSpec.describe 'nico:sync' do
     }.to change(PostVersion, :count).by(1)
 
     expect(post.reload.external_tags).to be_empty
-    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(TagName.where(tag_id: post.tags.select(:id),
+                         language_code: 'ja', primary_flg: true).pluck(:name))
+        .not_to include('bot操作')
   end
 
   it 'nico: prefix を含む従来の規則で外部タグ名をサニタイズする' do
diff --git a/backend/spec/tasks/post_similarity_calc_spec.rb b/backend/spec/tasks/post_similarity_calc_spec.rb
index 1934f66..721be6c 100644
--- a/backend/spec/tasks/post_similarity_calc_spec.rb
+++ b/backend/spec/tasks/post_similarity_calc_spec.rb
@@ -6,10 +6,10 @@ RSpec.describe 'post_similarity:calc' do
 
   it 'calculates similarities from active tags only' do
     # 必要最低限のデータ
-    t1 = create(:tag, name: 't1')
-    t2 = create(:tag, name: 't2')
-    t3 = create(:tag, name: 't3')
-    deprecated_tag = create(:tag, name: 'deprecated', deprecated_at: Time.current)
+    t1 = create(:tag, primary_name: 't1')
+    t2 = create(:tag, primary_name: 't2')
+    t3 = create(:tag, primary_name: 't3')
+    deprecated_tag = create(:tag, primary_name: 'deprecated', deprecated_at: Time.current)
 
     p1 = Post.create!(url: "https://example.com/1")
     p2 = Post.create!(url: "https://example.com/2")
diff --git a/backend/spec/tasks/tag_similarity_calc_spec.rb b/backend/spec/tasks/tag_similarity_calc_spec.rb
index fe293a6..a5aae72 100644
--- a/backend/spec/tasks/tag_similarity_calc_spec.rb
+++ b/backend/spec/tasks/tag_similarity_calc_spec.rb
@@ -6,10 +6,10 @@ RSpec.describe 'tag_similarity:calc' do
 
   it 'calculates similarities for active tags only' do
     # 必要最低限のデータ
-    t1 = create(:tag, name: 't1')
-    t2 = create(:tag, name: 't2')
-    t3 = create(:tag, name: 't3')
-    deprecated_tag = create(:tag, name: 'deprecated', deprecated_at: Time.current)
+    t1 = create(:tag, primary_name: 't1')
+    t2 = create(:tag, primary_name: 't2')
+    t3 = create(:tag, primary_name: 't3')
+    deprecated_tag = create(:tag, primary_name: 'deprecated', deprecated_at: Time.current)
 
     p1 = Post.create!(url: "https://example.com/1")
     p2 = Post.create!(url: "https://example.com/2")