But if the author translates their post into English, then French speakers still have to translate it back. The same problem happens.

No matter what you do, some people can't read what you post. Your choice is who you want to read it, and they probably don't care if Anglophones can read their post.

Wrong attitude, with that logic every post should use the absolute simplest vocabulary even while writing the same language. The author shouldn't expect everyone to use a translator, but those people can just scroll past. It's not up to the author to write for everyone.

Yes, though I think it only works with the first community mentioned, so you cannot spam a post across many communities that way.

Lemmy occasionally gets posts from people who do this unintentionally. This here is probably an example.

Belgium and Switzerland have French-speaking regions too and French is spoken by many Africans too, though you are right those aren't as widely connected to the Internet.