"Language without formalization is unreliable; "

What makes you believe LLMs will stick to the rules of your formalized language? It's likely. There are only probabilistic guarantees, which kind of obviates the benefits of a formal spec.

If you need formalism, wrap it into a formal language for the formal parts. But assuming that a formal language spec prompt avoids probabilistic outcomes? Yes, that is missing it by a million miles. You're missing the strength of LLMs. (Which is unstructured input->semi structured output)