Good luck... I spent many an hour trying to get this to work once- but languages don't behave like styles (which can be easily changed) in my experience. I never got it working.
First it needs to be a plugin, not a template conditional, so <vb:if...> is the wrong type of code;
I would have thought something like this on global_bootstrap_init_start would work:
PHP Code:
global $vbulletin;
if ($GLOBALS['forumid'] == 32)
{
$vbulletin->userinfo['languageid'] = 1;
}
and a bunch of variations on that, but never got it right.