IMO, that would be an interesting addy.. although, something like that would be a huge project but worth it. then there would be other languages to consider also, because if you do one there would be requests for others guarenteed! lol
I think I've seen scripts that had translation capabilities. I've never used them, so cannot tell you how they work or if they are any good.
Note that you can usually get free translation dictionaries online (edicts). Then you can use a $replace routine to do the translation. The problem with it is that you would be getting word-by-word translation rather than sentence translation. This is usually an incorrect way to translate text no matter what language you are using. Still, it would be close enough in most cases to be understood.