Trac ticket: https://core.trac.wordpress.org/ticket/43700

This continues the work on the patches already uploaded to 43700. It introduces a language switcher to the login screen, which also affects its derivatives such as the password reset screens and user registration.

The following closely related functionality has also been implemented:

• Add the language parameter to the login and password reset URLs in the email sent to a user when they are added to a site.
• Ensure the password reset email is always sent to the user in their chosen language.
• Add the language parameter to the URL in the password reset email.

Still to do:

• Decide whether, upon login, the user's locale should be updated to the one they chose on the login screen. Probably yes.