Function negotiate_locale 

pub fn negotiate_locale(
    preferred: &[String],
    available: &[String],
    default_locale: &str,
) -> String

Given a list of preferred locales (from Accept-Language) and a list of available locales (directories on disk), returns the best match.

Matching rules:

1. Exact match (e.g., “fr-CH” matches “fr-CH”)
2. Prefix match (e.g., “fr-CH” matches “fr”)
3. Default locale fallback

Examples

use ssg_i18n::negotiate_locale;

let pref = vec!["fr-CH".to_string(), "en".to_string()];
let avail = vec!["en".to_string(), "fr".to_string()];
assert_eq!(negotiate_locale(&pref, &avail, "en"), "fr");