One of the most important tools you will be using as a programmer is the programming language. Seems obvious, right? But it’s worth saying that.
The more in-depth knowledge you have about the language you use, the more productive and efficient you are.
I’m very biased towards the JavaScript language. It’s the language of the Web, it’s grown tremendously in the past 10 years and it has also expanded in what it can do.
From a small utility language that ran inside browsers, it has expanded to broad server usage with Node.js, and it’s also used on mobile and desktop apps with great success.
I think it’s great to learn more than one language, too. A language defines its own little world. Knowing more languages lets you be more wise with your choices, and know more little worlds.