Not much, throughout my career I have learned that the best path is always to only use the officially supported SDK languages for production code.

All productivity or enjoyment using alternative languages gets lost in extra FFI layer, unavailable platform documentation, lack of support on the IDEs, poorer debugging, GUI tooling and code generation.

So I only use alternative languages to learn about new concepts and ways to improve my skillset, but in what concerns production code, the golden rule is 100% SDK languages.

Going back to your question, Java on UWP is only possible via the JVM provided by CodenameOne and I think it actually makes use of the Desktop Bridge, so that rules out Scala.

Rust still has a lot to catch up with C++ for using COM in a productive way, let alone .NET based languages.

UWP is basically what .NET should have been, if they had kept the original COM+ plans instead of going with the CLR. So any language targeting UWP needs to have seamless COM support, and ability to handle .NET metadata.