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.

app/controllers/concerns/localized.rb

@@ -17,11 +17,7 @@ module Localized
   end
 
   def default_locale
-    request_locale || env_locale || I18n.default_locale
+    request_locale || I18n.default_locale
-  end
-
-  def env_locale
-    ENV['DEFAULT_LOCALE']
   end
 
   def request_locale
@@ -29,12 +25,10 @@ module Localized
   end
 
   def preferred_locale
-    http_accept_language.preferred_language_from([env_locale]) ||
+    http_accept_language.preferred_language_from(I18n.available_locales)
-      http_accept_language.preferred_language_from(I18n.available_locales)
   end
 
   def compatible_locale
-    http_accept_language.compatible_language_from([env_locale]) ||
+    http_accept_language.compatible_language_from(I18n.available_locales)
-      http_accept_language.compatible_language_from(I18n.available_locales)
   end
 end