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Not to mention Java, R, Ruby, PHP, Scheme, and sh.

https://www.postgresql.org/docs/9.5/external-pl.html

Heck, there's even a pl/prolog out there, but it looks pretty old and I'm skeptical how useful it would be.

They're cool, but keep in mind that this often means you cannot run them on managed Postgres services.

Wow, I've been writing a lot of PL/pgSQL recently and did not know this.

Would these be drop in replacements for PL/pgSQL? Are there any performance tradeoffs to using one over another? Any changes in functionality (aka functions you can call)?

I personally have never run into whatever the limitations might be of pl/python, but I suppose you could make a disaster by trying to dynamically load conflicting symbols or something like that. But I have used numpy in stored procedures, so at least that works.

pl/sh also works wonderfully if you want to run a complex procedure in an isolated subprocess, but pl/sh is not part of the main postgresql distribution.