Resolve "Email templates are not translated to zh_CN"

upstream MR

What does this MR do and why?

This MR is to solve the problem that the mail sent is not localized.

Bug reason: An around_action render_with_default_locale in ApplicationMailer used default locale instead of recipient language preference.

  def render_with_default_locale(&block)
    Gitlab::I18n.with_default_locale(&block)
  end

Solution: Define mail_with_i18n in ApplicationMailer to set the locale according to the recipient's language preference.Also replace mail with mail_with_i18n

  def mail_with_i18n(headers = {}, &block)
    locale = if headers[:to].is_a?(String) && !headers[:to].include?(',')
               User.find_by_any_email(headers[:to])&.preferred_language || I18n.locale
             else
               I18n.locale
             end

    Gitlab::I18n.with_locale(locale) do
      mail(headers, &block)
    end
  end

There are some reasons do this:

Our intention was to rewrite mail in ApplicationMailer:

• Why override mail: There are about 50 direct calls to mail (excluding spec) in gitlab. It would be very complex and difficult to maintain to set the locale on its caller without overriding mail
• Why override under ApplicationMailer: The callers of the mail method are all from the ApplicationMailer class and its subclasses. Overriding the mail method under ApplicationMailer can solve all problems at once

But it is inappropriate for mail to be rewritten as the base api. So we choose to define mail_with_i18n to achieve a similar effect. Even this adds modification: replace mail with mail_with_i18n.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.