There is no easy way to do that. You either need a bunch of new tables or else new fields in the existing tables.
If you really need multi-language support, I'd recommend that you look at the Community Server project from Telligent. They've already done the work for you. You can get all of the source code, and database objects for free, but you have to buy a license to use it commercially. It's worth your time just to study their database tables to get some ideas, even if you don't want to use their whole project.