fix(i18n): respect implicit and zero Accept-Language quality values in preferredLocale

.changeset/fix-preferred-locale-quality-sort.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fixes `Astro.preferredLocale` and `Astro.preferredLocaleList` ignoring `Accept-Language` quality values when they are absent or `0`. An entry without an explicit `q=` now correctly counts as quality `1.0` (per RFC 7231) and an entry with `q=0` is treated as not acceptable, so the highest-quality locale is selected regardless of header order.

packages/astro/src/i18n/utils.ts

@@ -71,10 +71,11 @@ function sortAndFilterLocales(browserLocaleList: BrowserLocale[], locales: Local
 			return true;
 		})
 		.sort((a, b) => {
-			if (a.qualityValue && b.qualityValue) {
-				return Math.sign(b.qualityValue - a.qualityValue);
-			}
-			return 0;
+			// An absent `q=` means quality 1.0 (RFC 7231), while a bare `*` defaults
+			// to 0 so it is never ranked above a real locale.
+			const qa = a.locale === '*' ? (a.qualityValue ?? 0) : (a.qualityValue ?? 1);
+			const qb = b.locale === '*' ? (b.qualityValue ?? 0) : (b.qualityValue ?? 1);
+			return qb - qa;
 		});
 }
 

packages/astro/test/units/i18n/i18n-utils.test.ts

@@ -67,6 +67,20 @@ describe('computePreferredLocale', () => {
 		assert.equal(computePreferredLocale(req, locales), 'fr');
 	});
 
+	it('prefers an implicit q=1 entry over a lower explicit-q entry regardless of header order', () => {
+		const req = new Request('http://example.com/', {
+			headers: { 'Accept-Language': 'en;q=0.7, de' },
+		});
+		assert.equal(computePreferredLocale(req, ['en', 'fr', 'de']), 'de');
+	});
+
+	it('excludes a q=0 entry from winning', () => {
+		const req = new Request('http://example.com/', {
+			headers: { 'Accept-Language': 'de;q=0, en;q=0.5' },
+		});
+		assert.equal(computePreferredLocale(req, ['en', 'fr', 'de']), 'en');
+	});
+
 	it('returns undefined when no match', () => {
 		const req = new Request('http://example.com/', {
 			headers: { 'Accept-Language': 'de,ja' },