English is by default the universal language. It's the official language of more countries than any other, and more people speak English than any other language. (Forget Mandarin. While it's the first language of more people than any other, more people speak English as a second language than any other.)
World wide, it's the language of science, business, and industry. A French pilot landing at an Italian airport converses in English with the control tower.
Knowing I was going to Mexico on a trip, I spent some time brushing up on my Spanish. I really looked forward to trying it out by speaking to the locals. I was disappointed though, since virtually everyone I met spoke better English than I did Spanish.
There is a huge demand to learn English in other parts of the world. If someone in Japan or Brazil or Nigeria is studying a language, it's almost certainly English.