@nixCraft But there are jobs for all popular languages.
Besides, when you already know 3-4 languages (backend, frontend, DB, scripting), you're pretty much set, the next best programming language to learn is either something fun or something very different from what you use, to broaden your way of thinking.

For instance, if you only know dynamic typed languages, learn a static, strongly typed language, and see the benefits of strong type systems. If you only know object oriented programming, learn a pure functional language, and see how you can apply some of the principles such as reducing side effects to better your code.

Most skills are not linked to a language, but learning different things force you to look into better ways to do what you're use to, and you can apply it to your favorite language.