But how can the Accept-Language header be useless? I suppose almost nobody sets this header by hand and it is derived from whatever environment your browser is

But how can the Accept-Language header be useless? I suppose almost nobody sets this header by hand and it is derived from whatever environment your browser is in. I mean if my Windows or my phone/tablet is set to German, showing me a German UI, wouldn't then the browser use that setting for Accept-Language? I can't understand how this would produce unreliable results unless there were some serious bugs in browser support (hopefully in the past).