Make Rails' 4.2 ActionMailer locale aware or i18n for ya emailz

actionmailer_i18n_monkeypatch.rb

module ActionMailer
  class MessageDelivery
    private
      def enqueue_delivery(delivery_method, options={})
        args = @mailer.name, @mail_method.to_s, delivery_method.to_s, I18n.locale.to_s, *@args
        ActionMailer::DeliveryJob.set(options).perform_later(*args)
      end
  end
  class DeliveryJob
    def perform(mailer, mail_method, delivery_method, locale, *args) # :nodoc:
      I18n.with_locale(locale) do
        mailer.constantize.public_send(mail_method, *args).send(delivery_method)
      end
    end
  end
end

Fixed in rails/rails#20800