Add global `locale` param (#17464)

- Remove the session-based locale stickyness

app/controllers/concerns/localized.rb

@@ -7,27 +7,24 @@ module Localized
     around_action :set_locale
   end
 
-  def set_locale
-    locale   = current_user.locale if respond_to?(:user_signed_in?) && user_signed_in?
-    locale ||= session[:locale] ||= default_locale
-    locale   = default_locale unless I18n.available_locales.include?(locale.to_sym)
-
-    I18n.with_locale(locale) do
-      yield
-    end
+  def set_locale(&block)
+    I18n.with_locale(requested_locale || I18n.default_locale, &block)
   end
 
   private
 
-  def default_locale
-    if ENV['DEFAULT_LOCALE'].present?
-      I18n.default_locale
-    else
-      request_locale || I18n.default_locale
-    end
+  def requested_locale
+    requested_locale_name   = available_locale_or_nil(params[:locale])
+    requested_locale_name ||= available_locale_or_nil(current_user.locale) if respond_to?(:user_signed_in?) && user_signed_in?
+    requested_locale_name ||= http_accept_language if ENV['DEFAULT_LOCALE'].blank?
+    requested_locale_name
+  end
+
+  def http_accept_language
+    HttpAcceptLanguage::Parser.new(request.headers.fetch('Accept-Language')).language_region_compatible_from(I18n.available_locales) if request.headers.key?('Accept-Language')
   end
 
-  def request_locale
-    http_accept_language.language_region_compatible_from(I18n.available_locales)
+  def available_locale_or_nil(locale_name)
+    locale_name.to_sym if locale_name.present? && I18n.available_locales.include?(locale_name.to_sym)
   end
 end

config/application.rb

@@ -149,10 +149,14 @@ module Mastodon
       :'zh-TW',
     ]
 
-    config.i18n.default_locale = ENV['DEFAULT_LOCALE']&.to_sym
-
-    unless config.i18n.available_locales.include?(config.i18n.default_locale)
-      config.i18n.default_locale = :en
+    config.i18n.default_locale = begin
+      custom_default_locale = ENV['DEFAULT_LOCALE']&.to_sym
+
+      if config.i18n.available_locales.include?(custom_default_locale)
+        custom_default_locale
+      else
+        :en
+      end
     end
 
     # config.paths.add File.join('app', 'api'), glob: File.join('**', '*.rb')
@@ -169,7 +173,6 @@ module Mastodon
       Doorkeeper::Application.send :include, ApplicationExtension
       Doorkeeper::AccessToken.send :include, AccessTokenExtension
       Devise::FailureApp.send :include, AbstractController::Callbacks
-      Devise::FailureApp.send :include, HttpAcceptLanguage::EasyAccess
       Devise::FailureApp.send :include, Localized
     end
   end