#428

backend/app/controllers/application_controller.rb

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