Switch locale to admin locale when sending admin notifications

[benniledl]

This patch takes the locale of the user associated with the admin_email and falls back to the site language if there is no user associated with this email address.

This patch adds a new funciton called get_admin_locale it takes the locale of the user associated with the admin_email and falls back to the site language if there is no user associated with this email address.

Also the patch switches to the get_admin_locale function in the admin email notifications instead of get_locale

Trac ticket: https://core.trac.wordpress.org/ticket/61518

[swissspidy]

Related: #49971

[benniledl]

Do you think we can milestone this to 6.7? @swissspidy

@at-benni I just pushed some changes to your branch to also update wp_notify_postauthor() and wp_notify_moderator to do the same thing. They're a bit more complex as they loop through a bunch of emails, but they would also benefit from switching to the correct locale per recipient.

We might want to do a single restore_current_locale() instead of doing restore_previous_locale() in a loop, for performance reasons.

In addition to manual testing, this change would benefit from some unit tests. At least wp_new_user_notification has some test coverage already, so we can start there.