Trying to go the Spec -> LLM route is just a lost cause. And seems wasteful to me even if it worked.

LLM -> Spec is easier, especially with good tools that can communicate why the spec fails to validate/compile back to the LLM. Better languages that can codify things like what can actually be called at a certain part of the codebase, or describe highly detailed constraints on the data model, are just going to win out long term because models don't get tired trying to figure this stuff out and put the lego bricks in the right place to make the code work, and developers don't have to worry about UB or nasty bugs sneaking in at the edges.

With a good 'compilable spec' and documentation in/around it, the next LLM run can have an easier time figuring out what is going on.

Trying to create 'validated english' is just injecting a ton of complexity away from the area you are trying to get actual work done: the code that actually runs and does stuff.