Across different paradigms, sure. But many languages are very similar to one another while only differing a bit in syntax.

If you use a language with a similar paradigm, learning yet another language in that paradigm will not teach you much, it'll mostly just annoy you with its subtle differences (I'm talking to you, stored and mutable default variables of python functions).

Python and JavaScript are at very different ends of the OOP spectrum. JS encourages a functional approach while Py is more procedural. That small difference can - and does - have a profound effect of who you structure your code.

These different designs will teach you different techniques for problem solving that can be borrowed in different languages when you’re trying to solve a problem that isn’t easily solved using its classic idiom.

I’ve never seen two languages so similar there was no didactic value to learning both. Hell I’ve never seen any language that offered nothing unique.

Except perhaps Java and early C#. before Linq.

That's true if learning meaningfully different languages, but much less so if it's a 5th imperative OO language.

Python and JavaScript have vastly different idioms, and learning both makes you think different. Learn both still makes you a better programmer, even considering they're both OOP languages

Like someone with keys to the world should spend time living in both Scotland and England to absorb the vast cultural differences?

Living in both? Certainly not. However I would advocate visiting Scotland if you’re English. It’s a beautiful country.

Likewise nobody says you need to be a full time developer in every language to get benefit from learning them.

The original comment that is evidently so objectionable mentioned keeping a language (and by implication its libraries) in one's head. It's a known fact that this type of knowledge is use-it-or-lose-it. So drawing on it only occasionally is terribly suboptimal.

I’ve not found that to be the case even after learning probably close on two dozen different languages.

I won’t deny that there is a short upskilling time where you need a quick refresher but that’s no different to picking up old code you’ve not touched in a while in a language you do use regularly. In fact often the time you spend familiarising yourself with an existing codebase or new API also serves as a catch up for the language too.

That said, I have personally found that it gets easier the more languages you learn because you get good at spotting similarities. Much like with spoken languages: people who are multilingual adapt to new languages quicker because they spot patterns to stuff they’ve learned previously.

You’re probably trying to be flippant but you’ve actually made an interesting point.

Americans and Canadians, for example, usually find uncanny-valley differences between their nations that get washed out if you visit somewhere radically different instead. The small-magnitude “it could’ve been something else” realizations are just as important.