Set the default locale in config (#6580)

Previously the default locale was set by Localized concern for controllers,
but it was not enforced for mailers.

config is enforced throughout the application and an appropriate place to
set the default locale.

config/application.rb

@@ -31,7 +31,7 @@ module Mastodon
     # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.
     # config.time_zone = 'Central Time (US & Canada)'
 
-    # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
+    # All translations from config/locales/*.rb,yml are auto loaded.
     # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
     config.i18n.available_locales = [
       :en,
@@ -73,7 +73,12 @@ module Mastodon
       :'zh-TW',
     ]
 
-    config.i18n.default_locale = :en
+    config.i18n.default_locale = ENV['DEFAULT_LOCALE']&.to_sym
+    if config.i18n.available_locales.include?(config.i18n.default_locale)
+      config.i18n.fallbacks = [:en]
+    else
+      config.i18n.default_locale = :en
+    end
 
     # config.paths.add File.join('app', 'api'), glob: File.join('**', '*.rb')
     # config.autoload_paths += Dir[Rails.root.join('app', 'api', '*')]

config/environments/test.rb

@@ -51,6 +51,9 @@ Rails.application.configure do
 
   # Raises error for missing translations
   # config.action_view.raise_on_missing_translations = true
+
+  config.i18n.default_locale = :en
+  config.i18n.fallbacks = true
 end
 
 Paperclip::Attachment.default_options[:path] = "#{Rails.root}/spec/test_files/:class/:id_partition/:style.:extension"