I've heard that if you're a good programmer, you should be able to pick up a new language in a few days. And I've done this myself, I learned the basics of Go a few months ago. Built some command line programs and a web app with it, all in a few days.
Yet, what does it take to really learn a new language? To master it? Sure, there are some concepts that translate well from language to language, but some languages have their own thing going on. It usually takes a little longer to even learn the basics of those languages.
To go beyond the basic concepts, you have to invest a lot of time into the language. You have to learn how to program idiomatically in it. I wouldn't go as far as saying that you have to spend 10,000 hours practicing a language, but I do think you have to spend at least a thousand hours to know it really well.
So after learning the language and learning all these tools, you must be a master in this language, right? Well, you're wrong. You see, most languages (even the unpopular ones) have something else associated with it: a community. Sometimes the community is really visible (for example, the Python community).
If you want to truly master a language, you should be part of its community. The community influences the language in many ways. Usually, the community influences the tools, new language features, libraries, frameworks, and so on. So if you have invested yourself into a language, you should also become part of the community.
Now you have accomplished all of the above: you have spent quite a few hours programming in the language, you have a broad knowledge of all its tools, and you're part of the community. Is that enough? No! You have to keep up to date, learning (or proposing) new features, tools, learning new libraries and frameworks. You could even build some of those yourself!