I18N: Add language switcher on login/registration screens.

Load a language switcher on the login and registration screens that allows users to choose any already-installed language. Set user locale on registration.

Props johnbillion, Nikschavan, afercia, sabernhardt, garrett-eclipse, keyur5, paaljoachim, Clorith, tobifjellner.
Fixes #43700.
Built from https://develop.svn.wordpress.org/trunk@52058


git-svn-id: http://core.svn.wordpress.org/trunk@51650 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -3044,6 +3044,13 @@ function register_new_user( $user_login, $user_email ) {
 
 	update_user_meta( $user_id, 'default_password_nag', true ); // Set up the password change nag.
 
+	if ( ! empty( $_COOKIE['wp_lang'] ) ) {
+		$wp_lang = sanitize_text_field( $_COOKIE['wp_lang'] );
+		if ( in_array( $wp_lang, get_available_languages(), true ) ) {
+			update_user_meta( $user_id, 'locale', $wp_lang ); // Set user locale if defined on registration.
+		}
+ 	}
+
 	/**
 	 * Fires after a new user registration has been recorded.
 	 *