[Logician]

You dont need to install 2 different vb (=2 licences) or 2 different database. If you can hack your vb, you can apply a simple trick:

Create 2 different usergroups in your board one for English users and one for Spanish. For each usergroup configure forum permissions so that they can only see forums in that language they are in.

Put new users into English group first and create a simple link in header which will change the usergroup of English User's to Spanish Group. If they are in Spanish Group, the link should change their group back to English. Now they will only see the forums in their own language unless they click that link and change their language (in fact their usergroup in background).

Easy isn't it?