there is in lines 2655-2658

elseif ($type == 'core')
	{
		return $cfg['lang_dir']."/$default/$name.$default.lang.php";
	}

so the language change doesn't work, i think there should be something like this

elseif ($type == 'core')
	{
		if (@file_exists($cfg['lang_dir']."/$lang/$name.$lang.lang.php"))
		{
			return $cfg['lang_dir']."/$lang/$name.$lang.lang.php";
		}
		else
		{
			return $cfg['lang_dir']."/$default/$name.$default.lang.php";
		}
	}

but i might be wrong...., as i said i'm not so good in php