It doesn’t actually make sense given the setting.
The Belters were subjugated, but not isolated. This is shown with a lot of the “security forces” largely being Earth/Mars based but also just general trade. Much like with the very large physiological differences (mostly in the books and season 1 of the show), ~100 years (since the Epstein Drive was invented 129 years before the protomolecule) just isn’t really enough for that kind of divergence.
The “oriental fetishism” of Firefly/Serenity is a lot more likely with mostly being a hybrid of different languages based on who the original colonists were… and that was a much larger period of time with even more isolation which is just funny.
Also, this ignores regular communication and media sharing between The Belt and The Inner Planets. Like, there have been some interesting studies that point out how pervasive “california english” is becoming because of “hollywood” (which films everything in Toronto). So even the Belters who more or less live in a mine their entire life would likely still watch documentaries from Oliver Queen’s baby mama and whoever Mars’s equivalent of Kim Kardashian is.
A lot of it boils down to the idea behind what a translation actually means.
One school of thought is that you do word for word translations. So you get very strange sentence structures and direct translations of idioms. Think the idea of (apologies to the French, it has been over a decade since I tried to write or speak any of it) “Il est drole” being translated to “He is to be funny” rather than “He is funny”.
The other school of thought is that you care about the meaning of the text and not the word for word translation. So that involves a LOT of updating/adapting idioms but also names. Among the weeb crowd, people lose their god damned minds any time a character has their name changed. But you get into a mess where “Hikari” is not meant to be an “exotic” name and is really being used closer to “Fred”.
I remember way back in high school we specifically read a version of Beowulf to demonstrate this. It involved the “original” Old English, a direct translation that is somehow even harder to parse, and then a version written in modern english.
And that is the basic idea. It doesn’t matter if Basic is English, Chinese, Russian, or (most likely) a hybrid of them all: What matters is that the reader/viewer understands what is going on and can appreciate the references.
Another example that usually comes up is how the movie A Knight’s Tale is one of the most “historically accurate” adaptations ever. Because yes, we have crowds of peasants singing Queen songs and Nike swishes on armor. But… that is a lot closer to what tournaments and jousting were than people playing equally inaccurate classical music. And I’ve made similar arguments for the god awful Romeo+Juliet where Luigi and Leo wield “sword 9mm” pistols.