Allow <lang> tags in forum subjects.

And filter them is $CFG->filterall is on

mod/forum/post.php

@@ -20,7 +20,7 @@
             $errordestination = $SESSION->fromurl;
         }
 
-        $post->subject = strip_tags($post->subject);  // Strip all tags
+        $post->subject = strip_tags($post->subject, '<lang>');        // Strip all tags except lang
         $post->message = clean_text($post->message, $post->format);   // Clean up any bad tags
 
         $post->attachment = $_FILES["attachment"];