Pretty much every language I've learned, I've learned on my own and only possibly later gotten paid for using.
For example, I picked up Elixir (which I currently make money using) by rewriting a Rails app into Phoenix four years ago. That app never made me any money, but it did provide a great motivation for learning Elixir!
Similarly, I'm learning Rust now. I'm going through books and videos, building toy projects and slowly picking it up. I've never been paid to write Rust, but that will likely change in the future.
Other languages, like Racket and 8088 assembly I've never been paid to write and likely never will but I don't regret exploring them a bit.