I spent about 30 minutes looking through templates and php files looking for a way to make the default font dependent on the language selected in vBulletin and couldn't even come up close...
Now you're saying you want to make it auto detect what language the user has set on their computer... this MIGHT be possible via PHP as explained here:
http://techpatterns.com/downloads/ph..._detection.php
But I still can't come up with code that would change your default font even if the language detection did work (and I'm not sure it will since it says it returns an array of all languages available to the browser, not the currently selected one.)