Currently just Scala and Python (via jep). Looking to add more (probably starting with Java and clojure) but haven't had time yet. There's just two of us working on it so far. PRs welcome!
Have you looked at GraalVM to bring in more languages?
Yep, we will be adding a plug-in to support Graal languages (little bit of learning to do on Graal first). We didn't want to make the project depend on Graal, though, because it's a pretty small segment of users (and we're not using it on our team at this point).
To verify, will this plugin allow external parties to add support to Polynote for more esoteric languages?
I don't want to over-promise anything given that I still have some reading to do about Graal's inner workings. But even given the interpreter side, there's also some plumbing to do (e.g. Monaco integration) that we haven't thought out yet. It's still in its infancy, and we'll need some help to be able to add stuff like this.
Get in touch with me if you want any help - I have experience in embedding Graal.