diff --git a/frontend/src/core/i18n/cookies.ts b/frontend/src/core/i18n/cookies.ts
index 255163dcc2..80e896bb80 100644
--- a/frontend/src/core/i18n/cookies.ts
+++ b/frontend/src/core/i18n/cookies.ts
@@ -15,9 +15,21 @@ export function getLocaleFromCookie(): string | null {
 
   const cookies = document.cookie.split(";");
   for (const cookie of cookies) {
-    const [name, value] = cookie.trim().split("=");
-    if (name === LOCALE_COOKIE_NAME) {
-      return decodeURIComponent(value ?? "");
+    // Split on the first "=" only: a cookie value may contain "=" itself.
+    const separator = cookie.indexOf("=");
+    if (separator < 0) {
+      continue;
+    }
+    if (cookie.slice(0, separator).trim() !== LOCALE_COOKIE_NAME) {
+      continue;
+    }
+    const value = cookie.slice(separator + 1).trim();
+    try {
+      return decodeURIComponent(value);
+    } catch {
+      // A malformed value must not crash locale bootstrap; hand the raw
+      // value to normalizeLocale, which falls back to the default locale.
+      return value;
     }
   }
   return null;
diff --git a/frontend/src/core/utils/datetime.ts b/frontend/src/core/utils/datetime.ts
index 030065d99b..e8d1a4bcc8 100644
--- a/frontend/src/core/utils/datetime.ts
+++ b/frontend/src/core/utils/datetime.ts
@@ -1,7 +1,7 @@
 import { formatDistanceToNow } from "date-fns";
 import { enUS as dateFnsEnUS, zhCN as dateFnsZhCN } from "date-fns/locale";
 
-import { detectLocale, type Locale } from "@/core/i18n";
+import { detectLocale, normalizeLocale, type Locale } from "@/core/i18n";
 import { getLocaleFromCookie } from "@/core/i18n/cookies";
 
 function getDateFnsLocale(locale: Locale) {
@@ -15,9 +15,9 @@ function getDateFnsLocale(locale: Locale) {
 }
 
 export function formatTimeAgo(date: Date | string | number, locale?: Locale) {
+  const savedLocale = locale ?? getLocaleFromCookie();
   const effectiveLocale =
-    locale ??
-    (getLocaleFromCookie() as Locale | null) ??
+    (savedLocale ? normalizeLocale(savedLocale) : null) ??
     // Fallback when cookie is missing (or on first render)
     detectLocale();
   // Guard against invalid/empty timestamps (e.g. a backend returning "" for
diff --git a/frontend/tests/unit/core/i18n/cookies.dom.test.tsx b/frontend/tests/unit/core/i18n/cookies.dom.test.tsx
new file mode 100644
index 0000000000..3bfb99f075
--- /dev/null
+++ b/frontend/tests/unit/core/i18n/cookies.dom.test.tsx
@@ -0,0 +1,26 @@
+import { afterEach, expect, test } from "@rstest/core";
+
+import { getLocaleFromCookie } from "@/core/i18n/cookies";
+
+function setLocaleCookie(value: string) {
+  document.cookie = `locale=${value}; path=/`;
+}
+
+afterEach(() => {
+  document.cookie = "locale=; max-age=0; path=/";
+});
+
+test("decodes an encoded locale", () => {
+  setLocaleCookie("zh-CN");
+  expect(getLocaleFromCookie()).toBe("zh-CN");
+});
+
+test("does not throw on a malformed percent-encoding", () => {
+  setLocaleCookie("%E0%A4%A");
+  expect(getLocaleFromCookie()).toBe("%E0%A4%A");
+});
+
+test("keeps a cookie value that contains '='", () => {
+  setLocaleCookie("en=US");
+  expect(getLocaleFromCookie()).toBe("en=US");
+});
diff --git a/frontend/tests/unit/core/utils/datetime.dom.test.ts b/frontend/tests/unit/core/utils/datetime.dom.test.ts
new file mode 100644
index 0000000000..c66049cd3a
--- /dev/null
+++ b/frontend/tests/unit/core/utils/datetime.dom.test.ts
@@ -0,0 +1,17 @@
+import { afterEach, expect, test } from "@rstest/core";
+
+import { formatTimeAgo } from "@/core/utils/datetime";
+
+afterEach(() => {
+  document.cookie = "locale=; max-age=0; path=/";
+});
+
+test("falls back to a known locale for a malformed locale cookie", () => {
+  document.cookie = "locale=zh%FF; path=/";
+  expect(() => formatTimeAgo(Date.now() - 5_000)).not.toThrow();
+});
+
+test("uses the locale from the cookie", () => {
+  document.cookie = "locale=zh-CN; path=/";
+  expect(formatTimeAgo(Date.now() - 5_000)).toContain("前");
+});