You go to a website, your browser sends it's local language, they ignore that entirely and just send whatever language they think you want because of geolocation, how are people so detached from the real world? In many cases (looking at you Google!) dont even let you change it.
It's OK to get Google to guess where I'm living at so that I get Google News for my country and Google Maps centered around my general location. I'm not logged in into Google so they have to guess.
If I go to a site selling some products it's much better if I get their view for my country, with the prices in our currency, than the US site with prices in dollars. Product availability could be different as well. I could switch and sometimes I have to because some sites guess that I'm American because they look at the language of my browser.
To sum up: interface in English, content in my language if available.
However there are many combinations of personal preferences and the place where we momentarily or permanently live. Websites have a hard time doing the right choice without the user explicitly telling them what to do. That often requires the creation of an account or the creation of a cookie, possibly management of personal data and unneeded complications.
That's what the browser sends a language headed for, except barely anyone respects it and just assumes, example from last night:
I am in CZ, my native language is English, my browser sends en-us, however Google not only switch between CS and DE ať random (presumably due to other travelers coming from the same IP address, I was on a train) but several of their auth flows etc don't offer the option to change language. So in this case it was wrong twice, and many native Czechs also use English as their interface choice, it's essentially telling the user they're dumb and in the other case, forcing people to navigate without being able to comprehend the page they're looking at.