url-mime-language-string

url-mime-language-string is a variable defined in `url-vars.el'.
Its value is

nil

Documentation:
String to send in the Accept-language: field in HTTP requests.

Specifies the preferred language when servers can serve documents in
several languages. Use RFC 1766 abbreviations, e.g.: `en' for
English, `de' for German. A comma-separated specifies descending
order of preference. The ordering can be made explicit using `q'
factors defined by HTTP, e.g. `de,en-gb;q=0.8,en;q=0.7'. `*' means
get the first available language (as opposed to the default).

You can customize this variable.