AcceptLanguage

Accept-Language is an HTTP header used by clients to indicate the preferred languages for the response. It informs the server of the user’s language and regional preferences so that the server can perform content negotiation and return a representation in an appropriate language when available.

The header value is a prioritized list of language ranges and optional quality values. Language tags follow BCP 47, such as en, en-US, fr-CA. Each tag can be assigned a q-value, a decimal between 0 and 1 that expresses preference; if omitted, the default is 1. Example: Accept-Language: en-US,en;q=0.9,fr;q=0.8. A wildcard can be used as a fallback with *; for instance, Accept-Language: en,*;q=0.5 means English is preferred, with any other language allowed at lower priority.

Server behavior centers on content negotiation. The server compares the request’s language ranges against available representations and selects the best match using q-values. If no acceptable variant exists, the server may respond with 406 Not Acceptable or default to its configured language. The response may include Content-Language to indicate the actual language of the payload, and Vary: Accept-Language to guide caches that the response depends on that header.

Practical use notes: user agents (browsers, apps) typically populate Accept-Language from user preferences and system locale. Developers can influence behavior by providing localized resources and by using appropriate Content-Language and Vary headers. Accept-Language is optional; its absence means the server should return its default language. Proxies and intermediaries may alter or ignore the header.