I realize it's sometimes not considered cool to point out competing projects, but the OneLang project FAQ explicitly says that it's a proof of concept, not ready for production use, with no documentation, and may never be finished.

https://github.com/koczkatamas/onelang/wiki/FAQ

OneLang does seem like a nifty project, but given its current state, it seems pretty reasonable to point toward Haxe for anyone interested in something similar that has been around for years, well documented, and used in production by many teams.

How so? Haxe is the de facto king of the compile-to-everything space. Therefore, it's inevitable that Haxe will be mentioned on a submission like this.

also consider https://blog.plan99.net/graal-truffle-134d8f28fb69

> Truffle is a framework for writing interpreters with annotations and small bits of extra code in them which, when Truffle is paired with its sister project Graal, allow those interpreters to be converted into JIT compiling VMs … automatically.

> If Truffle is a framework for writing AST interpreters, then Graal is the thing that makes them fast.

> Truffle provides a language interop framework called Polyglot that allows Truffle languages to call each other

> Haxe is the de facto king of the compile-to-everything space.

Indeed, I remember stumbling across it years ago while looking for a way to develop Flash games without Adobe Flash Professional. I never got around to actually try it but I keep seeing it every now and then, even saw a couple of books about it in a physical bookstore some years ago. It was the first thing that popped to mind for me as well when I saw that OP title mentioned write code in multiple languages at the same time.

What's the problem? It's a related project for those people who are interested in this sort of thing.