For example the language of the sea, and of the air is English. For example when a French or Chinese air traffic controller is communicating with an aircraft of their own nationality it must be in English, so that all the traffic which is listening on the same channel knows what is going on.
A similar thing has happened for international trade. I am British but work for a Norwegian International company. The official language including in Oslo head office is English. I was on a course in South Korea with a Norwegian instructor, and Korean, Chinese, Malay and Japanese attending. I was the only native English speaker, but everything was conducted in English.
I asked one of the Chinese students about how many learned English in China (he was from Shanghai). He said everyone, (but not everyone learns well of course).
I suppose another factor amongst those on the course is that they view English as a neutral language, so that it feels more equitable for a Korean to be negotiating with a Chinese person in English than Mandarin