Users: Correct the value of the `lang` attribute in the admin area.

This corrects the value when the user's language is set to `English (United States)` but the site language is not.

Props ocean90, afercia

See #42242

Merges [42220] to the 4.8 branch.

Built from https://develop.svn.wordpress.org/branches/4.8@42262


git-svn-id: http://core.svn.wordpress.org/branches/4.8@42091 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/general-template.php

@@ -715,7 +715,7 @@ function get_bloginfo( $show = '', $filter = 'raw' ) {
 			 */
 			$output = __( 'html_lang_attribute' );
 			if ( 'html_lang_attribute' === $output || preg_match( '/[^a-zA-Z0-9-]/', $output ) ) {
-				$output = get_locale();
+				$output = is_admin() ? get_user_locale() : get_locale();
 				$output = str_replace( '_', '-', $output );
 			}
 			break;