> Interestingly, Google does respect the Accept-Language header.
Google knows I use/want English. It knows this because I'm logged-in and have set my preferences, but also because of that Accept-Language header my browser sends in every single request. Yet for years, perhaps because I am 20km from Germany, it has assumed I would always prefer German when doing social sign-in.
Google may occasionally glance at Accept-Language, but it does not respect it. It even demotes my own logged-in preferences below a guess based on my location.
When there are no other indicators, fine, guess my language from my location, but otherwise location != language. But I know for fact that my browser is sending out Accept-Language headers in a convenient format that will tell you all you need to know.
Small thing / pet peeve: the website sets language based on my location rather than on what my browser is telling it is my language of choice in every single HTTP request.