I agree. If most of your experience is with a popular OO language, consider learning a lisp. Learn an ML. Try elixir/erlang to get a feel for how different programming for the BEAM can be.

I suppose you don't _have_ to use different languages to learn new concepts, but it certainly helps in some cases. For example, learning about currying is going to be much more natural in F# than it would be in C#.