Switch to admin user's locale when sending emails

[at-benni]

Description:

This patch uses the locale of a user associated with the admin_email and falls back to the site language if there is no user associated with this email address, when sending notifications to the administrator.

What issue does this solve?

1. Currently when a new user is registered the email notification to the admin is sent in the site locale, this patch tries to figure out which language the admin understands and sends the notification is that language.
2. Currently when a user changes his password, the email notification to the admin is sent in whatever locale the user, who is changing his password, uses. This patch also tries to figure out which language the admin understands and sends the notification is that language.

Trac ticket:

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

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[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.