Ticket #33511: 33511.3.patch

src/wp-includes/general-template.php

@@ -708 +708 @@
                         $output = $wp_version;
                         break;
                 case 'language':
-                        $output = get_locale();
-                        $output = str_replace('_', '-', $output);
+                        /* translators: Translate this to the correct language tag for your locale,
+                         * see https://www.w3.org/International/articles/language-tags/ for reference.
+                         * Do not translate into your own language.
+                         */
+                        $output = __( 'html_lang_attribute' );
+                        if ( 'html_lang_attribute' === $output || preg_match( '/[^a-zA-Z0-9-]/', $output ) ) {
+                                $output = get_locale();
+                                $output = str_replace( '_', '-', $output );
+                        }
                         break;
                 case 'text_direction':
                         _deprecated_argument( __FUNCTION__, '2.2', sprintf(