You post in your own language, and the site builds a translation for everyone, but they can also see your original etc.
I think building it as a forum feature rather than a browser feature is maybe worth.
We heavily use connected translating apps and it feels really great. It would be such a massive pita to copy every message somewhere outside, having to translate it and then back.
Now, discussions usually follow the sun, and when someone not speaking, say, Portuguese wants to join in, they usually use English (sometimes German or Dutch), and just join.
We know it's not perfect but it works. Without the embedded translation? It absolutely wouldn't.
I also used pretty heavily a telegram channel with similar setup, but it was even better, with transparent auto translation.
When I search for something in my native tongue it is almost always because I want the perspective of people living in my country having experience with X. Now the results are riddled with reddit posts that are from all over the world with crappy translation instead.
1. An automatic translation feature.
2. Being able to submit an "original language" version of a post in case the translation is bad/unavailable, or someone can read the original for more nuance.
The only problem I see with #2 involves malicious usage, where the author is out to deliberately sow confusion/outrage or trying to evade moderation by presenting fundamentally different messages.
It should be an intentional place you choose, and probably niche, not generic in topic like Reddit.
I'm also open to the thought that it's a terrible idea.
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.