refactor(accept-language): prefer the highest quality language

app/server/lib/__tests__/accept-language.test.ts

@@ -2,10 +2,18 @@ import { describe, expect, test } from "vitest";
 import { getLocaleFromAcceptLanguage } from "~/server/lib/accept-language";
 
 describe("getLocaleFromAcceptLanguage", () => {
-	test("strips quality values from the preferred language", () => {
+	test("strips quality values from a single preferred language", () => {
 		expect(getLocaleFromAcceptLanguage("en;q=0.5")).toBe("en");
 	});
 
+	test("prefers the highest quality language", () => {
+		expect(getLocaleFromAcceptLanguage("en;q=0.5, fr;q=0.9")).toBe("fr");
+	});
+
+	test("treats language tags without q as the highest priority", () => {
+		expect(getLocaleFromAcceptLanguage("en;q=0.9, fr-CH")).toBe("fr-CH");
+	});
+
 	test("skips invalid language tags and uses the next valid one", () => {
 		expect(getLocaleFromAcceptLanguage("*, en-GB;q=0.8")).toBe("en-GB");
 	});

app/server/lib/accept-language.ts

@@ -5,20 +5,48 @@ export function getLocaleFromAcceptLanguage(acceptLanguage?: string | null) {
 		return DEFAULT_LOCALE;
 	}
 
+	let locale = DEFAULT_LOCALE;
+	let highestQuality = -1;
+
 	for (const value of acceptLanguage.split(",")) {
-		const languageTag = value.split(";")[0]?.trim();
+		const [rawLanguageTag, ...parameters] = value.split(";");
+		const languageTag = rawLanguageTag?.trim();
 
 		if (!languageTag) {
 			continue;
 		}
 
+		let quality = 1;
+
+		for (const parameter of parameters) {
+			const [key, value] = parameter.split("=");
+
+			if (key?.trim().toLowerCase() !== "q") {
+				continue;
+			}
+
+			const parsedQuality = Number(value?.trim());
+
+			if (!Number.isFinite(parsedQuality) || parsedQuality < 0 || parsedQuality > 1) {
+				quality = -1;
+				break;
+			}
+
+			quality = parsedQuality;
+			break;
+		}
+
+		if (quality <= highestQuality) {
+			continue;
+		}
+
 		try {
-			Intl.getCanonicalLocales(languageTag);
-			return languageTag;
+			locale = Intl.getCanonicalLocales(languageTag)[0] || DEFAULT_LOCALE;
+			highestQuality = quality;
 		} catch {
 			continue;
 		}
 	}
 
-	return DEFAULT_LOCALE;
+	return locale;
 }