After you have some experience with, say, 3 to 5 different languages, you can learn new ones very quickly (hours, days) with little effort because you know almost all of the core concepts (OOP, FP, mutability, generics, ...). There's no point in learning "all of the languages" for its own sake. Just learn a bunch of different ones to learn different ideas and programming styles.

What takes more time is the standard library, common libraries, tools. A good example of that is iOS development in Swift. Learning the language is almost negligible compared to learning all of the technologies and tools. But coming from Android, I think I'm close to about 50 % of full productivity after two weeks.

I never understood why there's so much focus on the "x years experience in y" in the industry. A solid developer should be able to become fluent in any language and technology within a month or two.