Why do English become an international language?
English is a mish-mash of European languages, so even if you don’t know English, but are European, you can guess at what people are saying in English. England was an imperialistic country. That means that they traveled all around the world, taking over places. They have also been huge world traders. That means that they traveled all around the world buying and selling goods. Basically, the English people could be found all over the world. Look at all the countries in the world that were directly settled by the British: Canada, Australia, the U.S.A., New Zealand, … Then look at all the countries that have been controlled by the Brits: those same countries, plus India, Hong Kong, … Why are YOU learning English? Think about that. Think of all the reasons why you might be learning English now.
The British Empire once covered 1/6 of the Earth’s surface. The Brits mostly spoke English, and their political and economic influence was immense. In more recent times, the US has joined in, also with a huge economic influence, but also cultural ones, including movies. Put those two together, and you get a lot of English speakers. India alone has about 1 Billion. This influenced the rest of the world.
The British empire stretched over 1/3rd of the globe at one stage so English was taught on every continent, in its own context it was the super power of the day for over a century, if you wanted to trade you invariably had to deal with the Empire in one form or other, so the use of English grew until it became the international language.