Accept-Language Header — HTTP Content Negotiation

What Is Accept-Language?

The Accept-Language request header tells the server which languages the client prefers. It is sent automatically by browsers based on the user's OS and browser language settings.

Header Syntax

Accept-Language: <language-tag>[;q=<quality>], ...

The quality value (q) ranges from 0 to 1, with 1 being the highest preference (default when omitted).

Examples

Accept-Language: en-US
Accept-Language: en-US,en;q=0.9,ja;q=0.8
Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5

How Quality Values Work

Tag	Quality	Meaning
fr-CH	1.0 (default)	Most preferred: Swiss French
fr	0.9	Then any French
en	0.8	Then English
de	0.7	Then German
*	0.5	Then any language

Server-Side Handling

When a server receives this header, it should:

1. Parse the header into a sorted list of preferences
2. Match against available content languages
3. Return the best match with a Content-Language response header
4. Include a Vary: Accept-Language header for caching

// Node.js example
const acceptLanguage = req.headers["accept-language"];
// "en-US,en;q=0.9,ja;q=0.8"
// Parse and match against available locales

Relationship to BCP 47

Accept-Language values are BCP 47 language tags. The matching follows RFC 4647 "Matching of Language Tags," which defines both basic and extended filtering algorithms.

Privacy Considerations

The Accept-Language header can be used as a fingerprinting vector because it reveals the user's language preferences. Some privacy-focused browsers limit the information sent in this header.