AcceptLanguageHeader

Accept-Language header

Accept-Language is an HTTP header used by a client to announce preferred natural languages for the response. It participates in content negotiation, allowing servers to tailor content to the user’s language and locale when multiple representations are available.

Syntax and tags: The header lists one or more language ranges, defined by the IANA/BCP 47 language tag system. Each entry may include an optional quality value, indicated by q=, which ranges from 0 to 1 and defaults to 1 when omitted. Entries are comma-separated in order of preference. Example: Accept-Language: en-US,en;q=0.9,fr;q=0.8,ja;q=0.5. A star (*) may be used to indicate any language preferred by the client.

Negotiation and matching: The server compares the accepted language ranges to the language tags of available representations and returns the best match. If no appropriate variant is available, a fallback language or a default path may be used. Servers should consider producing a Vary: Accept-Language response header so caches know the response depends on the header.

Notes: The header is optional; many user agents set it for localization. It coexists with other mechanisms such as user profiles, cookies, and Accept headers. Standards are defined in RFC 7231 and related BCP 47 language tag specifications.