A website can receive language preferences from a browser, but those preferences do not establish the visitor’s nationality or location. Understanding the header helps separate a language choice from a personal inference.
Read what the header offers
The MDN Accept-Language reference describes a request header used in language negotiation. It can list preferred languages and relative priorities. A server may use that information when choosing a representation.
The browser’s preferences and the language of a particular page need not match. A user can deliberately read a foreign-language site or select a different language within the site itself.
Check explicit choices first
When investigating an unexpected language, record the browser preference, the site’s selected language and the resulting page. Do not assume the header is the only input to the decision.
For a site design, give people an understandable way to choose a language where alternatives exist. Technical negotiation can help with an initial presentation, but it should not become a reason to ignore an explicit selection. A language setting is useful because it expresses a preference, not because it supplies a reliable biography of the person behind the request.