I don't get the feeling this will happen. LLMs are extremely good at learning new languages because that's basically their whole point. If your new language has a standard library, and the LLM can see its source code, I am sure you can give it to any last-generation AI and it will happily spit out perfectly correct new code in it. If you give it access to a reference docs, then it can even ensure it never generates syntactically incorrect code quite well. As long as your error messages are enough to understand what a problem's root cause is, the LLM will iterate and explore until it gets it right.

Not sure if this is a good example, but I used ChatGPT (not even Codex) to fix some Common Lisp code for me, and it absolutely nailed it. Sure, Common Lisp has been around for a long time, but there's not so much Common Lisp code around for LLMs to train on... but OTOH it has a hyperspec which defines the language and much of the standard libraries so I believe the LLM can produce perfect Common Lisp based on mostly that.