English became the international language it is today because of the British Empire. About a quarter of the world's landmass was under British control at its peak and English was the official language in all those places. Since the former colonies became independent countries, it has generally remained people's first language or a widely used second language.
It helps that one of those former colonies, the US, has become very influential internationally and its culture has become very popular worldwide, which has helped spread the language further.