Comment by hahn-kev

As much as I agree Accept language should be enough, the problem is that English is the default value, so if it's default it's likely to be wrong so I understand guessing because most people don't know you can tell the browser what language you want to use. Partly to blame is also the websites that don't have other languages can't respect the header anyway. I've had plenty of users complain about localization issues and I have to explain that they can just tell the browser what language and region to use.