Function parse_accept_language 

pub fn parse_accept_language(header: &str) -> Vec<String>

Parses an Accept-Language header value into a sorted list of locale preferences (highest quality first).

Example: “fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5” Returns: ["fr-CH", "fr", "en", "de", "*"]