I don't have to, you can look at pretty much any of their language curriculum and find a huge presence of English in nearly all their education systems.

Certainly you will find people learning other languages for trade depending on the region, but even in East Asia, as you say, English is taught in China, Japanese, Korea. In Singapore English is the language everyone learns (and is taught in). In Vietnam the primary foreign language taught is English. In the Philippines one of its official languages is English. Argentina teaches English in elementary school. In Brazil students from grade 6 have to learn a language, which is usually English. In Venezuela English is taught from age 5.

So what exactly do I have to tell them?

Not sure about Latin America, but bring someone from each of China/Japan/Korea and they'll talk to each other in English.