Sci-fi tends to reduce all planets to monolithic cultures or two at best but it seems more likely each other planet would be as rich as our own with different languages, religions, cuisines etc
@anon_opin it’s maybe realistic depending on how difficult interplanetary travel is. Entire planets with a monoculture are just the natural extension of people wanting to live in a bubble of people who think the same as them, although it does raise some moral issues around any children brought up in those cultures believing it’s the only option.