feat: load app-only strings in every language, not just English

The RN overlay (`locales/rn/en.json`, the strings the webmail catalog
does not carry) only existed in English, so those 600 or so labels
showed in English whatever language the app was set to.

- load `locales/rn/<lang>.json` for every language next to its
  vendored catalog; an untranslated key still falls back to English
- a test keeps each translation to English keys and the same ICU
  arguments
- `node scripts/rn-missing-translations.mjs [lang]` lists what a
  language still lacks

locales/rn/ar.json

@@ -0,0 +1 @@
+{}

locales/rn/ca.json

@@ -0,0 +1 @@
+{}

locales/rn/cs.json

@@ -0,0 +1 @@
+{}

locales/rn/da.json

@@ -0,0 +1 @@
+{}

locales/rn/de.json

@@ -0,0 +1 @@
+{}

locales/rn/es.json

@@ -0,0 +1 @@
+{}

locales/rn/fa.json

@@ -0,0 +1 @@
+{}

locales/rn/fr.json

@@ -0,0 +1 @@
+{}

locales/rn/he.json

@@ -0,0 +1 @@
+{}

locales/rn/hu.json

@@ -0,0 +1 @@
+{}

locales/rn/it.json

@@ -0,0 +1 @@
+{}

locales/rn/ja.json

@@ -0,0 +1 @@
+{}

locales/rn/ko.json

@@ -0,0 +1 @@
+{}

locales/rn/lv.json

@@ -0,0 +1 @@
+{}

locales/rn/mn.json

@@ -0,0 +1 @@
+{}

locales/rn/nb.json

@@ -0,0 +1 @@
+{}

locales/rn/nl.json

@@ -0,0 +1 @@
+{}

locales/rn/pl.json

@@ -0,0 +1 @@
+{}

locales/rn/pt.json

@@ -0,0 +1 @@
+{}

locales/rn/ro.json

@@ -0,0 +1 @@
+{}

locales/rn/ru.json

@@ -0,0 +1 @@
+{}

locales/rn/sk.json

@@ -0,0 +1 @@
+{}

locales/rn/tr.json

@@ -0,0 +1 @@
+{}

locales/rn/uk.json

@@ -0,0 +1 @@
+{}

locales/rn/zh-TW.json

@@ -0,0 +1 @@
+{}

locales/rn/zh.json

@@ -0,0 +1 @@
+{}

scripts/rn-missing-translations.mjs

@@ -0,0 +1,51 @@
+// List the RN-only overlay keys (locales/rn/en.json) a language has not
+// translated in locales/rn/<lang>.json yet. Run from the RN repo root:
+//
+//   node scripts/rn-missing-translations.mjs        count per language
+//   node scripts/rn-missing-translations.mjs de     the missing keys with their
+//                                                   English text, as nested JSON
+//
+// Keys the vendored webmail catalog of that language already carries are not
+// listed: they are translated there.
+import { readFileSync, readdirSync } from 'node:fs';
+import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const ROOT = join(dirname(fileURLToPath(import.meta.url)), '..');
+const RN = join(ROOT, 'locales', 'rn');
+
+function flatten(obj, prefix = '', out = new Map()) {
+  for (const [k, v] of Object.entries(obj)) {
+    const key = prefix ? `${prefix}.${k}` : k;
+    if (v && typeof v === 'object' && !Array.isArray(v)) flatten(v, key, out);
+    else out.set(key, v);
+  }
+  return out;
+}
+
+function setDeep(obj, key, value) {
+  const parts = key.split('.');
+  let cur = obj;
+  for (const part of parts.slice(0, -1)) cur = cur[part] ??= {};
+  cur[parts.at(-1)] = value;
+}
+
+const readJson = (path) => JSON.parse(readFileSync(path, 'utf8'));
+
+function missingFor(lang, en) {
+  const done = flatten(readJson(join(RN, `${lang}.json`)));
+  const webmail = flatten(readJson(join(ROOT, 'locales', lang, 'common.json')));
+  return [...en].filter(([key]) => !done.has(key) && !webmail.has(key));
+}
+
+const en = flatten(readJson(join(RN, 'en.json')));
+const lang = process.argv[2];
+
+if (lang) {
+  const out = {};
+  for (const [key, value] of missingFor(lang, en)) setDeep(out, key, value);
+  console.log(JSON.stringify(out, null, 2));
+} else {
+  const langs = readdirSync(RN).filter((f) => f.endsWith('.json') && f !== 'en.json').map((f) => f.slice(0, -5));
+  for (const l of langs.sort()) console.log(`${l.padEnd(6)} ${missingFor(l, en).length} of ${en.size} missing`);
+}

src/i18n/__tests__/keys.test.ts

@@ -74,8 +74,8 @@ describe('translation coverage', () => {
   }, SRC_WALK_TIMEOUT);
 
   it('every other locale is a subset of English', () => {
-    // The vendored webmail catalog, without the RN-only overlay (which only
-    // exists for English and is served to every locale through the fallback).
+    // The vendored webmail catalog, without the RN-only overlay (English is
+    // complete; the other overlays are partial and fall back to it).
     const baseEn = flatten(
       JSON.parse(readFileSync(join(SRC_ROOT, '..', 'locales', 'en', 'common.json'), 'utf8')) as Record<string, unknown>,
     );

src/i18n/__tests__/rn-overlays.test.ts

@@ -0,0 +1,68 @@
+// The RN-only overlays of the other languages translate locales/rn/en.json.
+// Every key they carry must exist in English, be a non-empty string and use
+// the same ICU arguments, or t() would render a stale key or drop a value.
+// A key a language has not translated yet is fine: it falls back to English.
+import { describe, it, expect, vi } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+vi.mock('expo-localization', () => ({ getLocales: () => [] }));
+
+import { SUPPORTED_LOCALES } from '../index';
+
+const RN_DIR = join(__dirname, '..', '..', '..', 'locales', 'rn');
+
+function flatten(obj: Record<string, unknown>, prefix = '', out = new Map<string, unknown>()): Map<string, unknown> {
+  for (const [k, v] of Object.entries(obj)) {
+    const key = prefix ? `${prefix}.${k}` : k;
+    if (v && typeof v === 'object' && !Array.isArray(v)) flatten(v as Record<string, unknown>, key, out);
+    else out.set(key, v);
+  }
+  return out;
+}
+
+function readOverlay(code: string): Map<string, unknown> {
+  return flatten(JSON.parse(readFileSync(join(RN_DIR, `${code}.json`), 'utf8')) as Record<string, unknown>);
+}
+
+// Argument names a message uses: `{name}` and `{count, plural, …}`. The word
+// in a plural branch (`one {item}`) is text, not an argument.
+function argumentsOf(message: string): string[] {
+  const names = new Set<string>();
+  for (const m of message.matchAll(/(?<!\b(?:zero|one|two|few|many|other)\s*|=\d+\s*)\{\s*(\w+)\s*[,}]/g)) {
+    names.add(m[1]);
+  }
+  return [...names].sort();
+}
+
+describe('RN overlay translations', () => {
+  const en = readOverlay('en');
+
+  it('finds the arguments of plain and plural messages', () => {
+    expect(argumentsOf('Hello {name}')).toEqual(['name']);
+    expect(argumentsOf('{count, plural, one {# item} other {# items}} in {folder}')).toEqual(['count', 'folder']);
+    expect(argumentsOf('{count, plural, =0 {none} one {{count} day} other {{count} days}}')).toEqual(['count']);
+  });
+
+  for (const { code } of SUPPORTED_LOCALES) {
+    if (code === 'en') continue;
+    it(`${code} only translates English keys, keeping their arguments`, () => {
+      const problems: string[] = [];
+      for (const [key, value] of readOverlay(code)) {
+        const english = en.get(key);
+        if (typeof english !== 'string') {
+          problems.push(`${key}: not in locales/rn/en.json`);
+          continue;
+        }
+        if (typeof value !== 'string' || !value.trim()) {
+          problems.push(`${key}: empty or not a string`);
+          continue;
+        }
+        const want = argumentsOf(english).join(', ');
+        const got = argumentsOf(value).join(', ');
+        if (want !== got) problems.push(`${key}: arguments {${got}} instead of {${want}}`);
+      }
+      expect(problems).toEqual([]);
+    });
+  }
+});

src/i18n/index.ts

@@ -12,9 +12,9 @@ import { formatMessage, type MessageParams } from './format';
 // catalogs are loaded on first use; see CATALOG_LOADERS below.
 import en from '../../locales/en/common.json';
 
-// Keys the native app needs that the webmail catalog does not carry. Only an
-// English overlay exists today; other languages fall through to it via the
-// en fallback below.
+// Keys the native app needs that the webmail catalog does not carry. English
+// is complete; a key another language has not translated yet falls through
+// to it via the en fallback in translate().
 import rnEn from '../../locales/rn/en.json';
 
 export type { MessageParams } from './format';
@@ -71,10 +71,6 @@ function deepMerge(base: Dictionary, overlay: Dictionary): Dictionary {
   return out;
 }
 
-const RN_OVERLAYS: Partial<Record<LocaleCode, Dictionary>> = {
-  en: rnEn as Dictionary,
-};
-
 // A catalog is a few hundred KB of JSON. Metro only evaluates a module when it
 // is first required, so keeping each require inside its loader means startup
 // builds English plus the active language instead of all 27, and switching
@@ -110,15 +106,44 @@ const CATALOG_LOADERS: Record<LocaleCode, () => Dictionary> = {
   'zh-TW': () => require('../../locales/zh-TW/common.json'),
 };
 
+// The RN-only overlays, loaded alongside their catalog.
+const OVERLAY_LOADERS: Record<LocaleCode, () => Dictionary> = {
+  ar: () => require('../../locales/rn/ar.json'),
+  ca: () => require('../../locales/rn/ca.json'),
+  cs: () => require('../../locales/rn/cs.json'),
+  da: () => require('../../locales/rn/da.json'),
+  de: () => require('../../locales/rn/de.json'),
+  en: () => rnEn as Dictionary,
+  es: () => require('../../locales/rn/es.json'),
+  fa: () => require('../../locales/rn/fa.json'),
+  fr: () => require('../../locales/rn/fr.json'),
+  he: () => require('../../locales/rn/he.json'),
+  hu: () => require('../../locales/rn/hu.json'),
+  it: () => require('../../locales/rn/it.json'),
+  ja: () => require('../../locales/rn/ja.json'),
+  ko: () => require('../../locales/rn/ko.json'),
+  lv: () => require('../../locales/rn/lv.json'),
+  mn: () => require('../../locales/rn/mn.json'),
+  nb: () => require('../../locales/rn/nb.json'),
+  nl: () => require('../../locales/rn/nl.json'),
+  pl: () => require('../../locales/rn/pl.json'),
+  pt: () => require('../../locales/rn/pt.json'),
+  ro: () => require('../../locales/rn/ro.json'),
+  ru: () => require('../../locales/rn/ru.json'),
+  sk: () => require('../../locales/rn/sk.json'),
+  tr: () => require('../../locales/rn/tr.json'),
+  uk: () => require('../../locales/rn/uk.json'),
+  zh: () => require('../../locales/rn/zh.json'),
+  'zh-TW': () => require('../../locales/rn/zh-TW.json'),
+};
+
 const dictionaries: Partial<Record<LocaleCode, Dictionary>> = {};
 
 /** The merged catalog (vendored + RN overlay) for a locale, loaded on first use. */
 export function getDictionary(locale: LocaleCode): Dictionary {
   let dict = dictionaries[locale];
   if (!dict) {
-    const base = CATALOG_LOADERS[locale]();
-    const overlay = RN_OVERLAYS[locale];
-    dict = overlay ? deepMerge(base, overlay) : base;
+    dict = deepMerge(CATALOG_LOADERS[locale](), OVERLAY_LOADERS[locale]());
     dictionaries[locale] = dict;
   }
   return dict;