I believe HTTP has had this feature since 1999 as the Accept-Language header defined in the HTTP/1.1 RFC[0]. As for why it does not get used, MDN suggests[1] i

I believe HTTP has had this feature since 1999 as the Accept-Language header defined in the HTTP/1.1 RFC[0]. As for why it does not get used, MDN suggests[1] it's because changing it may lead to fingerprinting but there are likely other historical reasons. [0]: https://datatracker.ietf.org/doc/html/rfc2616#section-14.4 https://datatracker.ietf.org/doc/html/rfc2616#section-14.4 [1]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ac...