I've been working on some changes that might fix this. What you can try to do for starters is edit your newuser.php file and change line 12 from this:

require ("languages/$lang[THIS_FILE]/newuser.php");

to this:

require ("languages/" .$lang['THIS_FILE']. "/newuser.php");

If this fixes the newuser.php script then I'm on the right track so I'd appreciate a reply back if it fixes this one.