In my experience LLMs are way better when they "know" a language "instinctively". It's just that unless your language is very niche, the corpus is usually good enough. I tried using Claude to write my own personal language a while ago (I wrote a toy compiler decades ago) and it struggled a bit, because you could see in it's reasoning it had to "repeat" the syntax equivalence to itself while it read the code. It didn't just "know" it could use a given construct to do something; conversely Astra, when carefully instructed to do so, can plop down esoteric template code that works the first time, because it just "knows" it's the right stuff to write