/**
 * Pick the best match between a list of supported locales and a list of
 * requested locales, using BCP-47 best-match semantics:
 *
 *   1. Exact match — if any `requested` entry equals a `supported` entry
 *      after canonicalisation.
 *   2. Prefix match — if a `requested` tag is a parent of a `supported` tag
 *      (e.g. requested `pt` matches supported `pt-BR`).
 *   3. Parent match — if a `requested` tag's BCP-47 parents include a
 *      `supported` tag (e.g. requested `pt-PT` matches supported `pt`).
 *
 * **Casing:** comparison is case-insensitive — both `supported` and
 * `requested` tags are canonicalised internally (language lowercase, region
 * uppercase, script titlecase, per the convention browsers and `Intl.*`
 * emit). The **returned** locale preserves the original casing of the
 * matching `supported` entry — so `negotiateLocale(['PT-br'], ['pt-BR'])`
 * returns `'PT-br'`, not the canonical form.
 *
 * Returns the matching `supported` entry, or `undefined` if nothing matches.
 *
 * Pure utility — does not mutate `Ilingo` state. Compose with
 * `ilingo.setLocale(negotiateLocale(supported, requested) ?? defaultLocale)`.
 *
 * @example
 *   negotiateLocale(['en', 'pt-BR'], ['pt-PT', 'pt', 'en']);
 *   // → 'pt-BR'  (requested 'pt' is a parent of supported 'pt-BR')
 *
 *   negotiateLocale(['en', 'de'], ['fr', 'es']);
 *   // → undefined
 */
export declare function negotiateLocale(supported: string[], requested: string[]): string | undefined;
/**
 * Parse an HTTP `Accept-Language` header into an ordered list of locale tags,
 * sorted by quality (`q=`) descending. Tags lacking a `q=` parameter default
 * to `q=1.0` (RFC 9110).
 *
 * Tags with `q=0` are dropped per RFC 9110 § 12.5.4 — `q=0` means
 * "not acceptable", so a client that says `de;q=0` is explicitly rejecting
 * German and we must not negotiate to it. Invalid q-values (NaN or outside
 * `0..1`) cause the tag to be dropped as well.
 *
 * The `*` wildcard is dropped — callers that want a fallback should pass it
 * separately to `negotiateLocale`'s `requested` argument.
 *
 * @example
 *   parseAcceptLanguage('en-US,en;q=0.9,de;q=0.8');
 *   // → ['en-US', 'en', 'de']
 *
 *   parseAcceptLanguage('pt-PT;q=0.7, pt;q=0.5, *;q=0.1');
 *   // → ['pt-PT', 'pt']
 *
 *   parseAcceptLanguage('en, fr;q=0, de;q=0.8');
 *   // → ['en', 'de']    (French explicitly rejected)
 */
export declare function parseAcceptLanguage(header: string): string[];