There's a setting for language preference that is set in the browser, and transmitted in every request header. This is the thing that we're talking about.
Right, I have no idea how it works(and I guess like was said, 99% of people don't), but as a user I'd reasonably expect that if I have my browser in English, my operating system in English, then I want to see websites in English.
We don't need to know how it works, it asks the web pages to serve content in that language, it's made explicitly for this use case. It should be the first choice. Second choice could be browser language.
