What is a Content-Language header?
The natural language(s) of the intended audience for the response body.
Content-Language declares which human language (or languages) a website's response is actually written in, distinct from Accept-Language which is what the visitor asked for. Search engines and accessibility tools both use it to serve or announce content correctly, so a site that's meant to be multilingual but never sets this consistently is leaving both SEO and accessibility value on the table.
Common use case
A screen reader uses Content-Language to select the correct pronunciation and language rules for reading a page's text aloud to a visually impaired visitor.
Example
Content-Language: en-GBHistory
Part of the original HTTP/1.0 specification, RFC 1945 (1996).
Did you know?
It describes the content that was actually returned, not what the visitor asked for - a French-only site can legitimately send Content-Language: fr even to a visitor whose Accept-Language requested English.