My feeling is that it's very rare for people to configure their preferred language in the browser. It's a setting buried somewhere in the menus, and anecdotally

My feeling is that it's very rare for people to configure their preferred language in the browser. It's a setting buried somewhere in the menus, and anecdotally I would say most people don't even know such a setting exists. I suspect that, for the vast majority of people, their preferred language list is the default, which is then one entry, the same as the browser's UI language.