feat(i18n): add getMessageFallback so non-EN locales inherit EN keys (R5-6, D17)

src/i18n/request.ts

@@ -3,6 +3,19 @@ import { cookies, headers } from "next/headers";
 import { LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE } from "./config";
 import type { Locale } from "./config";
 
+/**
+ * Walk a nested messages object by a dot-separated path (namespace + key).
+ * Returns the found value as string, or undefined when the path does not exist.
+ */
+function getNestedValue(obj: Record<string, unknown>, namespace: string, key: string): string | undefined {
+  const ns = obj[namespace];
+  if (ns && typeof ns === "object" && !Array.isArray(ns)) {
+    const val = (ns as Record<string, unknown>)[key];
+    if (typeof val === "string") return val;
+  }
+  return undefined;
+}
+
 export default getRequestConfig(async () => {
   // 1. Try cookie
   const cookieStore = await cookies();
@@ -21,8 +34,40 @@ export default getRequestConfig(async () => {
 
   const messages = (await import(`./messages/${locale}.json`)).default;
 
+  // Always load EN so we can fall back to it when the active locale lacks a key.
+  // For EN itself the fallback is a no-op (same object), but the overhead is
+  // negligible since JSON imports are cached by the module loader.
+  const enMessages =
+    locale === DEFAULT_LOCALE
+      ? messages
+      : (await import(`./messages/${DEFAULT_LOCALE}.json`)).default;
+
   return {
     locale,
     messages,
+    getMessageFallback({
+      namespace,
+      key,
+    }: {
+      namespace: string | undefined;
+      key: string;
+      error: Error;
+    }): string {
+      // Try EN messages first.
+      if (namespace) {
+        const enValue = getNestedValue(
+          enMessages as Record<string, unknown>,
+          namespace,
+          key,
+        );
+        if (enValue !== undefined) return enValue;
+        // Return visible sentinel so QA notices missing keys.
+        return `${namespace}.${key}`;
+      }
+      // Top-level key (no namespace).
+      const topLevel = (enMessages as Record<string, unknown>)[key];
+      if (typeof topLevel === "string") return topLevel;
+      return key;
+    },
   };
 });