If you answer in the language you know best, it’ll be easier to others to understand or translate, especially if it’s English.

You could translate your message to match the language of the comment, but if you don’t know the language, how can you know if it conveys your message correctly?

Overall, I’d say it depends on the specific community. If you try to inject yourself into a conversation in a Japanese language community, it may indeed come off as rude or ignorant.

The best solution may be to post in both languages?

Most people just use the web interface

The thing about email is that the software is proprietary. Each of these providers has their own implementation of the interface, features, and integration with their tools (Google drive, photos, etc).

As long as lemmy servers run lemmy software, this won’t happen, or at least won’t be an issue as you can move to another server and not have to change your usage habits.

However if some server owners decide to fork Lemmy and develop their proprietary server, overhaul the UI, add features and attract users, it will start to become a problem.