My problem is I’m not able to make a simple list of all languages that I understand. Some I understand more, some less, depending on the subject. I’m sure I don’t even know the names of some of the languages I could make some sense of reading.
The default or original language of a page is what I want to see myself. Then, if it just looks like gibberish to me, I like the option of selecting an alternative language. And obviously I want to be able to share what I see with others.
The use of national flags is common, but those really denote political entities, not languages. I think it’s best to denote languages by their name, e.g. “English (US)”, but there are different UX aspects to this question.