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

Fixes #42242


git-svn-id: https://develop.svn.wordpress.org/trunk@42220 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/general-template.php

@@ -703,7 +703,7 @@ 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;