How To: Redirect with locale after authentication failure

Set your locale and make your default_url_options a class method in your ApplicationController

    def set_locale
      I18n.locale = params[:locale]
    end

    def self.default_url_options(options={})
      options.merge({ :locale => I18n.locale })
    end

Note: this differs from the Rails Internationalization (I18n) API Guide, but is required by the implementation of the Devise FailureApp (it calls ApplicationController.default_url_options(*args) in its default_url_options method).

Without this change, and if your routes are setup to require a locale, you will see errors like this on authentication failure: ActionController::UrlGenerationError (No route matches {:action=>"new", :controller=>"devise/sessions"} missing required keys: [:locale])