Show Chinese locale only when Chinese language signals are detected.

Hide the language switcher for English-only browsers; gate /zh via Accept-Language and a client cookie.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/chinese-locale.ts

@@ -0,0 +1,97 @@
+/**
+ * Detect whether the environment has Chinese language signals.
+ * Used to show/hide the 中文 UI option — English-only otherwise.
+ */
+
+const ZH_TAG = /^(zh|yue|cmn)([-_]|$)/i;
+export const ZH_OK_COOKIE = "cyd-zh-ok";
+
+export function isChineseLanguageTag(tag: string | null | undefined): boolean {
+  if (!tag) return false;
+  const primary = tag.trim().replace("_", "-");
+  return ZH_TAG.test(primary);
+}
+
+/** Parse Accept-Language / comma-separated locale list. */
+export function acceptLanguageHasChinese(
+  header: string | null | undefined
+): boolean {
+  if (!header) return false;
+  return header
+    .split(",")
+    .map((part) => part.split(";")[0]?.trim())
+    .some((tag) => isChineseLanguageTag(tag));
+}
+
+export type ChineseSignal =
+  | "navigator.language"
+  | "navigator.languages"
+  | "intl.locale"
+  | "accept-language"
+  | "cookie";
+
+export type ChineseDetection = {
+  likely: boolean;
+  signals: ChineseSignal[];
+};
+
+function readCookie(name: string): string | null {
+  if (typeof document === "undefined") return null;
+  const match = document.cookie.match(
+    new RegExp(`(?:^|; )${name.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}=([^;]*)`)
+  );
+  return match ? decodeURIComponent(match[1]) : null;
+}
+
+function writeCookie(name: string, value: string) {
+  if (typeof document === "undefined") return;
+  const maxAge = 60 * 60 * 24 * 365;
+  document.cookie = `${name}=${encodeURIComponent(value)}; Path=/; Max-Age=${maxAge}; SameSite=Lax`;
+}
+
+/**
+ * Client-side detection (browser / OS language prefs exposed to the page).
+ */
+export function detectChineseClient(): ChineseDetection {
+  const signals: ChineseSignal[] = [];
+
+  if (typeof navigator !== "undefined") {
+    if (isChineseLanguageTag(navigator.language)) {
+      signals.push("navigator.language");
+    }
+    if (navigator.languages?.some((l) => isChineseLanguageTag(l))) {
+      signals.push("navigator.languages");
+    }
+    const legacy = (navigator as Navigator & { userLanguage?: string })
+      .userLanguage;
+    if (isChineseLanguageTag(legacy) && !signals.includes("navigator.language")) {
+      signals.push("navigator.language");
+    }
+  }
+
+  try {
+    const intlLocale = Intl.DateTimeFormat().resolvedOptions().locale;
+    if (isChineseLanguageTag(intlLocale)) {
+      signals.push("intl.locale");
+    }
+  } catch {
+    /* ignore */
+  }
+
+  if (readCookie(ZH_OK_COOKIE) === "1") {
+    signals.push("cookie");
+  }
+
+  return { likely: signals.length > 0, signals };
+}
+
+/** Persist so middleware allows /zh after client-side Chinese detection. */
+export function markChineseLocaleAllowed() {
+  if (typeof window === "undefined") return;
+  try {
+    writeCookie(ZH_OK_COOKIE, "1");
+    window.localStorage.setItem(ZH_OK_COOKIE, "1");
+  } catch {
+    /* ignore */
+  }
+}