So, I may be wrong about this, but my guess is that a lot of non English speakers browsers are not sending the "Accept-Language" header with the correct language. Getting that language right requires configuration that most users can't/won't do, so it's only going to be right if the system defaults do that, and I think it's pretty likely that a lot of users will be getting the wrong language.
If the above is true, then it almost certainly makes more sense for these large international sites to just ignore the header and send the correct language for a given country. Some travelers may be inconvenienced, but a much larger number of users will benefit.
If the above is true, then it almost certainly makes more sense for these large international sites to just ignore the header and send the correct language for a given country. Some travelers may be inconvenienced, but a much larger number of users will benefit.