I’ve often wondered how exactly English came to be so dominant as a world language. The British deserve the credit for getting this started, but after that it’s a little fuzzy. Other countries, most notably Spain and France, had empires as well. The French language is spoken, at least by elites, in many countries, but most of these are former colonies. Spanish is spoken by a large number of people as well, but mostly as a first language. Yet English has by far the largest number of non-primary speakers, over a billion.
The long history of the British empire, as I said, got this started. But before they were dominant there was the Spanish empire, which now pales in linguistic influence. The relevant difference may be that Spain’s empire was (relatively) geographically concentrated, while Britain’s possessions were far-flung across the world.
One key component in the 20th century is the fact that two great-power nations used it. There are other great powers, but no two with a language in common. In a case of 2 vs. 1 vs. 1…, it looks like the language shared by two powers wins. (Austria-Hungary and Germany shared a language, but I have trouble classifying Austria-Hungary on the same level as the US and UK, and even within Austria-Hungary most people would have preferred speaking another language than German.)
I still don’t have a great answer, but a study mentioned in Die Welt today says that English is the most efficient language, so perhaps that’s another reason.