My view is that it's not just that English-language fandom is huge and it's not just that it's full of monolingual people but also that you have to read pretty widely to find fic you'd actually want to bother translating. (I mean, there's a ton of stuff in English that I liked but would never spend that kind of time on.) The only time I've ever tried translating fic was when someone specifically asked for my help translating hers.