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.
There should be some bit about the locale being default or not. If its not a default, then respecting it would be fine. If its the default, you could try guessing.
Websites can only guess the language until the user explicitly choses. Accept-Language is a good guess, but as you pointed out it often defaults to English, or it may transmit language codes the website doesn't support (It sends `es-MX`, but website only supports `es-ES`). The website still has to guess.
English is in fact my primary language. It may be the default, but I cannot set it to something else. I have explicitly chosen to leave it on the default, and would like websites to accept this.
There shouldn't be a default. By default, browsers should just not send the header, which would signal the web site to do whatever stupid thing its developers conjure up. But if the header is sent, it should override the server-side-stupidity. This would solve the "but by default it's English!" objection.
Currently Browsers default to sending the Browser/OS UI language. Which is an good default because users can understand that language (they are using that for the UI after all) unlike whatever your GeoIP provider says. The "but by default it's English!" objection never had any merit.
