TinyMCE: add lang attribute

This is needed to make `hyphens: auto;` work correctly.

Fixes #32555.

Built from https://develop.svn.wordpress.org/trunk@35217


git-svn-id: http://core.svn.wordpress.org/trunk@35183 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/tinymce/plugins/wordpress/plugin.js

@@ -428,6 +428,8 @@ tinymce.PluginManager.add( 'wordpress', function( editor ) {
 			dom.setAttrib( doc.documentElement, 'dir', 'rtl' );
 		}
 
+		dom.setAttrib( doc.documentElement, 'lang', editor.getParam( 'wp_lang_attr' ) );
+
 		if ( env.ie ) {
 			if ( parseInt( env.ie, 10 ) === 9 ) {
 				bodyClass.push('ie9');