As for cross platform, there are Element and FluffyChat. There is also nheko I can recommend for desktop and cinny on web.
If you don’t insist on matrix, Snikket might be a good option too.
It’s something close: https://discourse.mozilla.org/t/about-config-missing/66537/6
About:config is not a feature that most people will use and it can make your browser seriously unstable.
I think you mean language filter. Internalization often means something else
Mastodon tries to implement a language filter too, but it didn’t go very well from what I heard – false negative and false positive happen both too often.
I wouldn’t be against this feature though. IMO there should be a message confirming if the language is recognized correctly to refine the language model, but that might be diverting developers’ effort from more helpful features.
The client API can be used to make it possible. There is a mirror bot on lotide, btw
i don’t think it’s possible yet