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I can't tell you about The Expanse (other than that it's a great show), but the master of invented, fictional languages is J.R.R. Tolkien.
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Think there are linguistic experts who can do it. Though it may look super hard, it may not be that expensive as it’s becoming a thing in many works of fiction. Interestingly an Indian movie too attempted it recently.
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Did you read the books? They explain a lot about the language, and emphasize the sign language component a lot more. The spoken part follows a lot of similar rules as Jamaican dialects use. So, yes it's expensive world building, but it's not a whole new language.
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