ActionController and ActionMailer default include AbstractController::Rendering directly or indirectly (ActionController::Metal::Rendering). AbstractController::Rendering uses ActionView as default library for its template rendering engine and includes AbstractController::ViewPaths and an instance of I18n proxy to find localized views. To learn more i'd like to refer to the ActionPack source code on github. To get to the point. ActionView allows you to use localisation in your templates: See Rails guide: Action View Overview , Chapter Localized views.