Other JVM languages are technically allowed, but AFAIK they generate way too much scaffolding & reflection-based flow such that you end up burning your entire b

Other JVM languages are technically allowed, but AFAIK they generate way too much scaffolding & reflection-based flow such that you end up burning your entire bytecode computation budget just in calling one or two functions. So yes, you could probably use Jython or Jruby, but you wouldn't be competitive against the people writing straight Java.