English become a global language all thanks to one empire, the British Empire. With all the colonies they set up across the world, be it Asian or Middle-East, English is taught to the natives to ease their communication. Why not learn the native language? Simple really, the English looks down on them and would rather force the natives to learn their language. It is a colony after all, with the British as the master. But yeah, they never really did reinforce that the natives must only learn and speaks in English. English is just used as the language for business and trade. With that, a quarter of the world speaks something that resembles English.
Then we came to the greatest country in the world, the USA. In making it united, being made up of bunches of Europeans who speaks a variety of language, the founders decide on the English as the national language. So once again, English is forced upon those who do not want to learn it in the USA, with banning of teaching of foreign languages. Coincidently, the English language is a language that adopts words from other languages such as Latin, French and German. An empire monopolizes the market and the world, with a language that symbolize unity and learning, then it becomes the national language of the USA. The USA with its huge influence of the world would mean that other country would need to learn English to trade with them.