It was invented (for the TV) by linguist Nick Farmer
He used Haitian creole as his guide, because its speakers all came from elsewhere to work on the island. Slaves taken from Africa combined their own native languages with the dominant French, and the result was a shared tongue that only the slaves understood. Like Haiti, the Belt is dominated by a wealthy class of colonizers (in this case, Earthers who speak English). But all the labor is done by what Farmer calls "economic slaves" who risk their lives in mines.
The resulting Belter creole is a crazy mix of English, Chinese, romance languages like French, German, Persian, Hebrew, Zulu, and a few other surprises. Farmer says he has over 1,000 Belter words in his personal dictionary, and he keeps adding more as the show's producers and fans request them.
Nick Farmer knows dozens of languages, so he invented one for The Expanse
The language in the books series is apparently little more than gibberish, according to the co-author.
Daniel Abraham: For serious students, I strongly recommend focusing on the Belter creole from Nick Farmer and not putting too
much credence on the stuff in the books. Nick is a professional
linguist with a deeply rooted understanding of the project. What we're
doing in the book is less rigorous and done with a very different set
of constraints and goals.
BerserkHaggis: I was at your book signing/talk at Powell's Bookstore in Portland a couple years back right after Cibola Burn came
out, (you two were amazing) and I asked about the thought process that
went into the Belter Creole. Ty replied "We picked shit that sounded
cool" and you said "Yup!" :-D
Daniel Abraham: WE could pretend otherwise, but.... :)
Sasa ke Belter Creole? with Daniel Abraham