From my experience, I predict that LLM code completion will not work very well when using a language like Civet.
Copilot is good when working with something where the idiomatic patterns are common, widespread, and hard to screw up. Civet has low adoption, uses much looser syntax (bracing, whitespace), and lacks the rigor of common TypeScript code.
I was pleasantly surprised with how well Copilot picked it up. Civet doesn't have that many truly new language features, most of them are from existing languages and used in a similar way. Copilot is really good at matching what you are doing near the completion so I was impressed with how well it did with a new language.
Copilot is good when working with something where the idiomatic patterns are common, widespread, and hard to screw up. Civet has low adoption, uses much looser syntax (bracing, whitespace), and lacks the rigor of common TypeScript code.