Well, my point is, I don’t think a really divergent language would arise at all.
In Earth history, during the years of European expansion around the world, such local languages arose in several places, because many different people from different places ended up in one location, and had to come up with some sort of common language to just survive everyday life. Each locale developed their own version because they didn’t all have the same mix of original languages to start with. And, because travel times between locales was measured in weeks or months, with no way to send voice messages at all, there was very little pressure for these various languages to conform to one common, or at least similar, usage. Sailors would probably find it easier to talk several languages than to try to get everyone else to speak one language.
But when was the last time we saw such a language evolve? Now that everyone has easy access to TV, radio and the internet, we’re all starting to sound alike. I’ve seen this in my own family from Newfoundland, where people of my generation and younger have lost a lot of the Newfie accent that used to be so prominent, it was considered its own dialect, with lots of borrowings from French.
So I don’t see the Belters developing a language so different that people from Earth would actually need lessons to even just understand what was being said.
That being said, the one Belter thing I do like is the development of hand signs that can be used when in a suit in vacuum to communicate essential information, as we saw Naomi doing this week. That’s the sort of language development that would occur, as it would be essential for Belters, but virtually useless for Earthers. And while Mars would probably have something similar for surface work in suits, it makes sense that it would be different - some problems are the same, but others are not. Belters probably don’t have to worry about dust storms, for instance. Also, Belters would mostly be in zero G, any so could use a whole-body language, where leg or foot placement could hold meaning.