Over the years, I've worked with about 20 different languages. I learned a lot of them purely out of interest. Even if you won't need it for any "serious" or paying work, it can be useful to learn a new language that is different from the languages you know. For example, if you know C# you won't learn that much by working with Java; they're too similar. By contrast, if you try learning a language like Haskell or Go instead you'll get introduced to new ways of thinking.
In almost all languages, there are things you can do easily or "naturally" in it. These language (and framework) features usually influence how you would design a program in that language. And it's these concepts that are worth learning. For example, when I learned Ruby and later Haskell, I learned how powerful concepts like map/select or working with closures are.
This knowledge then transferred to the languages I usually work with; my designs in my "traditional" languages changed because of the things I learned while working with other languages.
So even if the new language is not "one the rise" it might pay off by indirectly improving your skills in the languages that you do get paid for.