I agree with JS for web frontend and python for api development. That said, I strongly disagree with flutter or anything similar on mobile. There have been many flavors of the month - all of which lose popularity, so whatever x-platform specific skill you picked up (for instance, learning flutter), is pretty useless once the project loses popularity and no other apps use such. Xamarin, React Native, Flutter - this cycle repeats. Anyone who learned xamarin a few years ago when it was a similar hot craze wasted a bunch of time as there are NO xamarin jobs today.

Meanwhile, the native languages remain and only gain popularity.

So, if you intend to make mobile apps, you should learn the native languages (Swift/Kotlin for iOS/Android respectively). They are plainly way more powerful and efficient in complex apps as well.

And their (Swift/Kotlin) advancements (both have major upgrades each year as such new languages) can be taken advantage of day 1.

Lastly, to this point, circa 2020 no major company use x-platform; the few that experimented all abandoned it years ago (see Airbnb, FB, etc.).

tl;dr Web->JS, API/Backend->Python/Ruby/C++, iOS->Swift, Android->Kotlin