Well, ideally, before accepting such a project, you have a list of mandatory things the clients wants, and send it to the client to double check it. That way, you know exactly what is coming your way, and whether you can do it or not. And if the client wants stuff that's not on the list, you can charge extra.
I'm not sure where your problem is. Is it the translation? Do you have the translated content? Or is it creating some button that lets the user choose the language?