How to access langcode in a (custom) transport?

Templated emails has a locale option, but they are not used by the module atm.

https://github.com/symfony/symfony/commit/d912384b7f2e46c5053e0f42514a9e...

https://symfony.com/doc/current/mailer.html#html-content

https://symfony.com/doc/current/mailer.html#adding-tags-and-metadata-to-...

Do $message arrays typically have a langcode key in them, or is that something that you're going to add on your site manually with hook_mail_alter or in the hook_mail? If so, I think you should be able to just add $message['headers']['X-Langcode'] right there, and it wouldn't require any changes to the module. I think it would only make sense to change the module if Drupal was automatically adding langcode and we're currently ignoring that.

Also, it appears there is an official header for language, and it's Content-Language: https://www.rfc-editor.org/rfc/rfc4021.html#section-2.2.10. So, if we add something to the module, we should probably try to use that.

https://github.com/composer/composer/discussions/12024

MailManagerInterface requires a langcode to be specified every time and the default MailManager implementation does add that to the $message payload: https://github.com/drupal/core/blob/10.3.0/lib/Drupal/Core/Mail/MailMana....

Also, it appears there is an official header for language, and it's Content-Language: https://www.rfc-editor.org/rfc/rfc4021.html#section-2.2.10. So, if we add something to the module, we should probably try to use that.

Good catch, thumbs up on that!

I am considering to open a Drupal core issue (over the weekend) for adding the Content-Language header to all emails sent by the mail manager...

Okay, I'm also fine with coding the header into this module, given that $langcode should always be present in the $message array. We should just have a check to make sure it's not LANGUAGE_NONE. In that case, we should skip the header.

Re #6, I cannot find the core issue. Did you file that? Would you mind linking to it from here?