With the way the market is, unless you stay at one job forever, its actually hard to stay with the same language... the world changes, opportunities change, your interests change...you'd have to try very hard to stick to one language.

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.