When you download vbulletin, there was a folder called "do_not_upload". In that folder is a file called tools.php. Put that in your admincp folder and then access it through your browser - ie.
http://www.yoursite.com/admincp/tools.php. Enter your customer number. Then select and run "Language" under "Import XML Files".
(I hope that's correct. I've never had to use that file. knock on wood.)