What is an Accept-Language header?
Tells the server which human languages, and in what order of preference, the client would like the response in.
Accept-Language carries the visitor's preferred languages, in ranked order, so a multilingual website can serve the right localisation without the visitor having to pick manually. It's worth testing deliberately if you run an uptime monitor against a site with region-specific content, since a bug in the language-negotiation logic can silently serve the wrong locale to an entire audience segment without ever producing an error status code.
Common use case
A multilingual website reads Accept-Language on first visit to redirect a visitor to their localised subdomain or path without requiring an explicit language picker.
Example
Accept-Language: en-GB,en;q=0.9,fr;q=0.8History
Part of the original HTTP/1.0 specification, RFC 1945 (1996).
Did you know?
Browsers typically build this list from the operating system's configured languages, so it often reveals a visitor's OS locale settings even if they never explicitly configured the browser itself.