I couldn't agree more, but if I could make a recommendation. If you already know a mainstream OO language, you won't get much benefit out of learning another in the same arena. Going from Java to C# you will learn less than going from javascript to ocaml or Java to Haskell.
The best thing (for my own learning) I ever decided to do was learn Haskell. I have never been paid to write code in Haskell but it gave me such a confidence with languages that I don't think I've seen any language features in any other language that has ever surprised me or that I felt like I couldn't learn. Haskell and it's language extensions will expose you to many many different ideas. It helped me learn Rust, I feel like I can read ocaml, any other functional language doesn't feel like a stretch to read, etc
So, don't just learn many languages. My advice would be to pick languages across paradigms and learn them. Don't waste your time learning 5 object oriented languages.
The best thing (for my own learning) I ever decided to do was learn Haskell. I have never been paid to write code in Haskell but it gave me such a confidence with languages that I don't think I've seen any language features in any other language that has ever surprised me or that I felt like I couldn't learn. Haskell and it's language extensions will expose you to many many different ideas. It helped me learn Rust, I feel like I can read ocaml, any other functional language doesn't feel like a stretch to read, etc
So, don't just learn many languages. My advice would be to pick languages across paradigms and learn them. Don't waste your time learning 5 object oriented languages.