Users: In edit_user() fall back to site's locale instead of en_US for invalid user locales.

See #38632, #29783.
Fixes #38798.
Built from https://develop.svn.wordpress.org/trunk@39269


git-svn-id: http://core.svn.wordpress.org/trunk@39209 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/user.php

@@ -100,8 +100,10 @@ function edit_user( $user_id = 0 ) {
 			$locale = sanitize_text_field( $_POST['locale'] );
 			if ( 'site-default' === $locale ) {
 				$locale = '';
-			} elseif ( ! in_array( $locale, get_available_languages(), true ) ) {
+			} elseif ( '' === $locale ) {
 				$locale = 'en_US';
+			} elseif ( ! in_array( $locale, get_available_languages(), true ) ) {
+				$locale = '';
 			}
 
 			$user->locale = $locale;