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/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"