test(i18n): guard message catalogue key parity across locales

src/i18n/messages.test.ts

@@ -0,0 +1,45 @@
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { describe, expect, it } from 'vitest';
+
+// Locale dictionaries are hand-maintained. English is the source of
+// truth (src/i18n/request.ts falls back to en.json only when a whole
+// locale file is missing — there is no per-key fallback), so a key
+// that lands in en.json and not in a translation renders as a raw
+// keypath for users on that locale. This guards the parity.
+
+const MESSAGES_DIR = join(process.cwd(), 'messages');
+const SOURCE_LOCALE = 'en';
+const TRANSLATED_LOCALES = ['ko'];
+
+function loadKeys(locale: string): Set<string> {
+  const raw = readFileSync(join(MESSAGES_DIR, `${locale}.json`), 'utf8');
+  const out = new Set<string>();
+  const walk = (node: unknown, path: string) => {
+    if (node && typeof node === 'object' && !Array.isArray(node)) {
+      for (const [k, v] of Object.entries(node)) {
+        walk(v, path ? `${path}.${k}` : k);
+      }
+      return;
+    }
+    out.add(path);
+  };
+  walk(JSON.parse(raw), '');
+  return out;
+}
+
+describe('message catalogue parity', () => {
+  const source = loadKeys(SOURCE_LOCALE);
+
+  it.each(TRANSLATED_LOCALES)('%s.json covers every en.json key', (locale) => {
+    const translated = loadKeys(locale);
+    const missing = [...source].filter((k) => !translated.has(k)).sort();
+    expect(missing, `${locale}.json is missing these keys`).toEqual([]);
+  });
+
+  it.each(TRANSLATED_LOCALES)('%s.json has no orphaned keys', (locale) => {
+    const translated = loadKeys(locale);
+    const orphaned = [...translated].filter((k) => !source.has(k)).sort();
+    expect(orphaned, `${locale}.json has keys absent from en.json`).toEqual([]);
+  });
+});