"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.