That's an interesting idea.
My first thought (that the same adapter code used by FFF to read sites could be pointed at Google's translated page) doesn't work. Google Translate doesn't generate the full page, it uses javascript to only translate each block of text as it comes into view. And it changes the HTML rather a lot.
I'm not interested in trying to implement translation in FFF, but I suspect that it's possible to implement a separate plugin using something like
googletrans that will translate an ebook.
I bet such a plugin could be quite popular if someone wanted to write one.