Rails emails using the recipient's locale.

notification_mailer.rb

class NotificationMailer < ActionMailer::Base

  default from: "Sender <sender@example.com>"

  def mail_with_locale(headers = {}, &block)
    locale = headers.delete(:locale) || I18n.default_locale
    I18n.with_locale(locale) { mail_without_locale(headers, &block) }
  end
  alias_method_chain :mail, :locale

  def notification(user)
    mail to: user.email, locale: user.locale
  end

end