i18n(ja): ship Japanese

- Register ja: CATALOGS, LOCALES, LOCALE_NAMES (日本語) and the package
  boundaries; ja and ja-JP tags select it.
- Japanese unit words follow the number directly, as in the catalog:
  formatQuantity writes 5分 and 1時間5分 (km and kt keep the space), and
  formatRelativeTime drops the space CLDR puts in 5 分前.
- Tests: locale tags, picker order, Intl output, durations and the
  ramp/exit/lane wording in Japanese.
- docs/translations/ja.md: style, units, plurals and glossary; ja added to
  TRANSLATORS.md and CURRENT-STATE.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/i18n/i18n.test.mjs

@@ -5,6 +5,7 @@ import {
   applyDocumentTranslations,
   formatDate,
   formatNumber,
+  formatRelativeTime,
   msg,
   persistLocaleAndReload,
   setLocale,
@@ -35,6 +36,9 @@ test('normalizeLocale folds regional variants and rejects unshipped tags', () =>
   assert.equal(normalizeLocale(' RU '), 'ru');
   assert.equal(normalizeLocale('en_GB'), 'en');
   assert.equal(normalizeLocale('xx'), null);
+  assert.equal(normalizeLocale('ja'), 'ja');
+  assert.equal(normalizeLocale('ja-JP'), 'ja');
+  assert.equal(normalizeLocale('JA_jp'), 'ja');
   // Explicit script tags take precedence over regional defaults.
   assert.equal(normalizeLocale('zh'), 'zh-CN');
   assert.equal(normalizeLocale('zh-CN'), 'zh-CN');
@@ -164,7 +168,13 @@ test('Intl formatters follow the active locale', () => {
   assert.equal(formatNumber(1234.5), '1,234.5');
   setLocale('ru');
   assert.equal(formatDate(date, options), '15 января 2026 г.');
+  assert.equal(formatRelativeTime(-5, 'minute'), '5 минут назад');
   assert.equal(formatNumber(1234.5), '1 234,5');
+  // Japanese keeps a number and its unit together, as the catalog does.
+  setLocale('ja');
+  assert.equal(formatDate(date, options), '2026年1月15日');
+  assert.equal(formatRelativeTime(-5, 'minute'), '5分前');
+  assert.equal(formatRelativeTime(-3, 'day'), '3日前');
 });
 
 function fakeElement({ text = '', attributes = {} } = {}) {

src/i18n/index.js

@@ -17,11 +17,18 @@ import {
   writeStoredLocale,
 } from './locale.js';
 import es from './locales/es.json' with { type: 'json' };
+import ja from './locales/ja.json' with { type: 'json' };
 import ru from './locales/ru.json' with { type: 'json' };
 import zhCN from './locales/zh-CN.json' with { type: 'json' };
 import zhHant from './locales/zh-Hant.json' with { type: 'json' };
 
-const CATALOGS = Object.freeze({ es, ru, 'zh-CN': zhCN, 'zh-Hant': zhHant });
+const CATALOGS = Object.freeze({
+  es,
+  ja,
+  ru,
+  'zh-CN': zhCN,
+  'zh-Hant': zhHant,
+});
 
 let currentLocale = DEFAULT_LOCALE;
 
@@ -201,13 +208,19 @@ export function formatUtcDayMonth(value) {
 
 /**
  * Locale-aware relative time (Intl.RelativeTimeFormat), e.g.
- * formatRelativeTime(-5, 'minute') → '5 minutes ago' / '5 минут назад'.
+ * formatRelativeTime(-5, 'minute') → '5 minutes ago' / '5 минут назад' /
+ * '5分前'.
  */
 export function formatRelativeTime(value, unit = 'second', options) {
-  return cachedFormatter('relative', Intl.RelativeTimeFormat, {
+  const text = cachedFormatter('relative', Intl.RelativeTimeFormat, {
     numeric: 'auto',
     ...options,
   }).format(value, unit);
+  // CLDR spaces Japanese (5 分前); the ja catalog writes the number and its
+  // unit together (5分前), so the two must not differ on one screen.
+  return currentLocale === 'ja'
+    ? text.replace(/(?<=\d) (?=\p{Script=Han})/gu, '')
+    : text;
 }
 
 /** Collapse markup whitespace so a message reads the same in HTML and JS. */

src/i18n/locale.js

@@ -8,12 +8,20 @@ export const LOCALE_STORAGE_KEY = 'gev:locale:v1';
 export const DEFAULT_LOCALE = 'en';
 
 /** Shipped locales; order does not matter. Adding one: docs/TRANSLATORS.md. */
-export const LOCALES = Object.freeze(['en', 'es', 'ru', 'zh-CN', 'zh-Hant']);
+export const LOCALES = Object.freeze([
+  'en',
+  'es',
+  'ja',
+  'ru',
+  'zh-CN',
+  'zh-Hant',
+]);
 
 /** Each locale named in its own language, for the selector. */
 export const LOCALE_NAMES = Object.freeze({
   en: 'English',
   es: 'Español',
+  ja: '日本語',
   ru: 'Русский',
   'zh-CN': '中文（简体）',
   'zh-Hant': '中文（繁體）',
@@ -23,7 +31,7 @@ export const LOCALE_NAMES = Object.freeze({
 const RTL_LOCALES = Object.freeze([]);
 
 /**
- * Map any language tag to a shipped locale: 'ru-RU' and 'RU' → 'ru',
+ * Map any language tag to a shipped locale: 'ru-RU' and 'RU' → 'ru', 'ja-JP' → 'ja',
  * 'zh', 'zh-CN' and 'zh-Hans-SG' → 'zh-CN';
  * 'zh-Hant', 'zh-TW', 'zh-HK' and 'zh-MO' → 'zh-Hant'.
  * @returns {string|null}