I get that the title didn't say much, but I really didn't know how to explain it

I am interested in cooperating with another forum in a way that when users register on that forum, they also register on my forum(and the other way around) and also posts they make should be also posted in a special forum for posts from that forum.I know this most likely doesn't exist, but I wanted to check, and if it doesn't exist as I think it doesn't, feel free to move my topic to suggestions

Not possible.

No sorry we would need server side to do that and we do not have the right to FM's servers

Well, I am looking for a way for users on my forum to be able to view everything in versions for other languages, so do you have some advice?

Create sections for different languages on your forum.

Yes, but will I be able to get a post from one section, copy it to another one and still have its post date and everything the same?

If you make it a global announcement, it will appear in all forums.  If you want one topic in only two forums, you will have to manually create them.