I usually ask myself if the language can do things that I can't do with a language I already know. For example, I learned a bit of Go because it shines at concurrency in a way that other languages I know didn't. My first language was C++ and I wanted to learn a scripting language so I took up Python. Etc., etc.

Beyond that, I prefer to learn languages that are well documented and have strong communities for support. In my opinion, these not only make it easier to learn the language, but are also good proxies for whether the language will continue to thrive.