At work I use C# - but I hope to keep learning more languages. The main thing I was worried about initially was being stuck with one language my whole career.
Anyone else ever feel that way?
Now, on that topic, what's much harder than learning multiple languages, is learning multiple language paradigms. If you have a C++ background, learning C#, Java, Swift, JavaScript, whatever is pretty easy.
Learning Haskell, ML, Elm, F#, Clojure, etc...now that will make your mind bend and spin for a bit. Those aren't nearly as common in a work environment, but you can gain a lot from learning at least one.
This is kind of similar to how if you're an english speaker, learning another european language isn't nearly as interesting as learning a CJK.
Being stagnant as a developer is something I never want to happen.
I would suggest picking a language you have an interest in and a project you have an interest in and having a go at it. Then repeat the process with a new language and a new project. At the end, you will have some working software and an expanded perspective.
Ideally I'd like to work on something others could make use of, but I've been having a hard time thinking of something interesting to work on.