[onion2k]

In my experience learning other languages makes you better at the ones you already know. You get exposure to different ideas, paradigms, and tools that you can bring back to your main language. For that reason I'd suggest looking at others.

[OskarS]

I agree, especially if the other language uses markedly different paradigms. Like, if you already know Java, learning C# is not going to massively expand your horizons, the languages are too similar.

Instead, you should try something like a functional language (like Haskell or a Lisp variant), or a dynamically typed language (like Python), or a language with a different memory management model (like C, C++ or Rust). It will give you practical skills and expand your horizons on how to solve problems with programming.