A while back I posted a template modification that translates threads using Google's Language Tools. It's not perfect, but neither are the translation services available out there right now. It doesn't do quite what you where wanting, but it's something you might want to look at.
DJ's Translate a Thread in a Google Pop-Up Window
EDIT: Looks like I need to add a few more languages to it. Google added a few more when I wasn't looking.