Racket is at least related to the kind of metalanguage system you're talking about. I've never actually done it, but to implement a new "#lang" in Racket, your

Racket is at least related to the kind of metalanguage system you're talking about. I've never actually done it, but to implement a new "#lang" in Racket, your job is essentially to write a "reader" for it that transliterates it to the classic Schemey Racket language. Libraries written in one #lang can then be called from one another (or not, if that's what you want -- a lot of the point of this in Racket is building little restricted teaching languages that have limited capabilities).