This is true, but it doesn't mean you should actually use a variety of languages for your day job if you're self-employed; that is, you lose some productivity if you choose an unfamiliar tool, and you'll shoot yourself in the foot if you choose a language unsuitable for the problem at hand.

The other thing to watch out for if you're in a corporate setting is that if you use a language nobody else is using at your company, your project is doomed and / or it will be stupid expensive compared to other projects because it uses a different language from the rest of the organization. See also https://boringtechnology.club/

I would say there are many, many other ways to broaden my perspective, not just learning and using a different programming language. There are so many things to learn!

(And I don't necessarily suggest not learning different languages, if one fancies that. I just don't use any other ones at the moment.)

I actually find it a bit of a curse to know too many languages. Because settling on one for any given project becomes very difficult. Like you start coding it up and then hit some friction and start thinking about one of the other languages you know that would be more elegant. Or you might have chosen a language that does a lot of things elegantly but then start thinking of the performance you've given up performance for that. Swapping languages will never stop those thoughts because there's always tradeoffs somewhere.