(http://www.vbulletin.com/forum/showthread.php?t=159359)
Try uploading the original install/vbulletin-language.xml file. Then upload tools.php to the install directory and run it from your browser (there is an option to import the language).
The tools.php file is in the do_not_upload folder in the vBulletin zip file.
^^ ^^
After you do that, create a new language with default parameters. That should work.