Ooo.. it wouldn't be a plugin. You would have to delete and reinsert thousands of lines in the databases, then rewrite all of the grammar code in the script, and even then I suspect it would only speak poor French. There are probably embedded routines that can't be adapted. And then almost all of the current plugins wouldn't work.
It would have to be a labor of love on Robert's part.
However, I bet if you just start speaking French to it, it will end up speaking a combination of English and French in a very short time. If you push the learning slider over some, it might even speak mostly French (but again, probably bad French...)