Comment by AcidBurn

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

[1]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ac...