I remember a company I worked with used to have its internet connection go through German servers, even though it was in France. I remember the presentation page for one of Google's phone was in German with no way to have it in anything else. Using a ".fr" domain, "accept-language", logged into a French Google account, I also tried English too, no luck. I only managed to get some UI elements in something other than German, it felt like a troll. I wonder how the Swiss (a small country with 4 official languages) are doing.
In addition to the idea that the IP address is a pretty bad indicator of the language you speak, some websites have trouble with the idea that people can understand more than one language.
Hey, YouTube, I know I have set my account to French, but I understand English and you know it, so please don't auto-translate video titles for me and do a terrible job at it. And I don't want to set my account to English, because I guess it would translate French titles into English, which I want even less.