HTTP Content Negotiation also has a simple protocol for negotiating the desired human language of the data sent back to the client. Clients use the Accept-Language header to specify which human language they would like to receive. For example:
GET http://example.com/stuff Accept-Language: en-us, es, fr
Here, the client is asking for a response in English, Spanish, or French. The Accept-Language header uses a coded format. Two digits represent a language identified by the ISO-639 standard.8 You can further specialize the code by following the two-character language code with an ISO-3166 two-character country code.9 In the previous example, en-us represents US English.
The Accept-Language header also supports preference qualifiers:
GET http://example.com/stuff Accept-Language: fr;q=1.0, es;q=1.0, en=0.1
Here, the client prefers French or Spanish, but would accept English as the default translation.
Clients and servers use the Content-Language header to specify the human language for message body translation.
9 For more information, see the ISO website.