If the site was constructed with a Content Management System such as PostNuke or PHPNuke, then there are language modules available. These modules allow users to translate the site into their native language.
Being only an English speaker, I have no idea how accurate these things are, but from what I have seen of foreign sites using these modules, the conversions to English are pretty good.
It would require a complete overhaul of the site, and the forums would have to be changed to a different system that ran as a plugin to either PostNuke or PHPNuke.
Both systems are relatively easy to implement, and there is a large community behind both projects. Personally I use PostNuke as I find it easier to setup and manage, but as always, YMMV.