[douglas]

English is the primary trade language of the world because Great Britain went on a gigantic colonization spree for a century or two and managed to outdo everyone else at it, and everywhere they went they took their language with them. At its height, the British Empire included the American colonies (which became the USA), India, Australia, huge swaths of Africa, and part of Canada. France got a share of Canada, and Spain took Mexico and maybe some parts of South America, but they never even came close to matching Great Britain.

I think I've seen some statistics that Spanish is actually spoken by more people than English, but English speakers are spread out more, so it's easier for non-speakers to find a translator in more places for English than for anything else.