The accept-language header is dead. It's simply ignored by every site. Some crappy sites like Origin even make it impossible to set the language to anything but German even by changing the URL directly. Google took years to even allow me to manually set everything to English, until about 1-2 years ago some settings were impossible for me to reach in English even when everything was set to English.

It's just like with Windows, applications use the location instead of the OS language to chose what language to default to.

L10n is broken. And no one seems to have any real interest in fixing it. The tools are there but they get ignored by pretty much everyone.