"unless you live in a bilingual neighborhood" is key here. Many countries across the world teach English as a second language and in western trade it's the lingua franca. Mandarin is spoken in various countries, but many of the Mandarin-speaking countries also have English as a common second language.

Going by [this list](https://en.wikipedia.org/wiki/List_of_languages_by_total_num...), you can see how if you want to expand your reach, some languages make more sense to learn than others.

Yes, which is why most languages are only spoken by native speaker.

No because English is the de-facto language for trade.

Functionally bilingual neighborhoods are, in fact, quite common.