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.