It would be a derivative work of the api, so per Oracle's argument you would be violating their copyright absent a license. (You could use the gpl OpenJDK license if your code was GPLed, I think).

You could try and argue fair use, and it would be distinct from the case at hand so you might win, but fair use is always a fact intensive analysis and it's certainly not obvious that you would win.

You'll need a license for that if APIs are now copyrightable.

Your toy language cannot read the API information into memory to compile with it, because that would be copying it.