diff --git a/app/server/lib/__tests__/accept-language.test.ts b/app/server/lib/__tests__/accept-language.test.ts
new file mode 100644
index 00000000..f1f29013
--- /dev/null
+++ b/app/server/lib/__tests__/accept-language.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, test } from "vitest";
+import { getLocaleFromAcceptLanguage } from "~/server/lib/accept-language";
+
+describe("getLocaleFromAcceptLanguage", () => {
+	test("strips quality values from the preferred language", () => {
+		expect(getLocaleFromAcceptLanguage("en;q=0.5")).toBe("en");
+	});
+
+	test("skips invalid language tags and uses the next valid one", () => {
+		expect(getLocaleFromAcceptLanguage("*, en-GB;q=0.8")).toBe("en-GB");
+	});
+
+	test("falls back to en-US when no valid language is present", () => {
+		expect(getLocaleFromAcceptLanguage(";;;, ???")).toBe("en-US");
+		expect(getLocaleFromAcceptLanguage(null)).toBe("en-US");
+	});
+});
diff --git a/app/server/lib/accept-language.ts b/app/server/lib/accept-language.ts
new file mode 100644
index 00000000..5f0ca86a
--- /dev/null
+++ b/app/server/lib/accept-language.ts
@@ -0,0 +1,24 @@
+const DEFAULT_LOCALE = "en-US";
+
+export function getLocaleFromAcceptLanguage(acceptLanguage?: string | null) {
+	if (!acceptLanguage) {
+		return DEFAULT_LOCALE;
+	}
+
+	for (const value of acceptLanguage.split(",")) {
+		const languageTag = value.split(";")[0]?.trim();
+
+		if (!languageTag) {
+			continue;
+		}
+
+		try {
+			Intl.getCanonicalLocales(languageTag);
+			return languageTag;
+		} catch {
+			continue;
+		}
+	}
+
+	return DEFAULT_LOCALE;
+}
diff --git a/app/server/lib/functions/root-loader-data.ts b/app/server/lib/functions/root-loader-data.ts
index 00f7f599..7f788f6f 100644
--- a/app/server/lib/functions/root-loader-data.ts
+++ b/app/server/lib/functions/root-loader-data.ts
@@ -2,6 +2,7 @@ import { createServerFn } from "@tanstack/react-start";
 import { getCookie, getRequestHeaders } from "@tanstack/react-start/server";
 import { THEME_COOKIE_NAME } from "~/client/components/theme-provider";
 import type { DateFormatPreference, TimeFormatPreference } from "~/client/lib/datetime";
+import { getLocaleFromAcceptLanguage } from "~/server/lib/accept-language";
 import { auth } from "~/server/lib/auth";
 
 export const getRootLoaderData = createServerFn({ method: "GET" }).handler(async () => {
@@ -12,7 +13,7 @@ export const getRootLoaderData = createServerFn({ method: "GET" }).handler(async
 
 	return {
 		theme: (themeCookie === "light" ? "light" : "dark") as "light" | "dark",
-		locale: (acceptLanguage?.split(",")[0] || "en-US") as string,
+		locale: getLocaleFromAcceptLanguage(acceptLanguage),
 		timeZone: process.env.TZ || Intl.DateTimeFormat().resolvedOptions().timeZone || "UTC",
 		dateFormat: (session?.user.dateFormat ?? "MM/DD/YYYY") as DateFormatPreference,
 		timeFormat: (session?.user.timeFormat ?? "12h") as TimeFormatPreference,