I think the audience that would be interested in this is vanishingly small, there exist relatively few conversations online that would be meaningfully improved by this.

I also suspect that automatically translating a forum would tend to attract a far worse ratio of high-effort to low-effort contributions than simply accepting posts in a specific language. For example, I'd expect programmers who don't speak any english to have on average a far lower skill level than those who know at least basic english.