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/data/directions.test.mjs

@@ -875,6 +875,13 @@ test('route numbers follow the page language and never break from their unit', (
   );
   const list = directionsStepList({ route });
   assert.equal(list.items[1].lead, '240\u00a0м');
+
+  // Japanese unit words follow the number: 1時間5分, but 1.5 km.
+  setLocale('ja');
+  assert.equal(routeDistanceText(1500), '1.5\u00a0km');
+  assert.equal(routeDurationText(39), '39秒');
+  assert.equal(routeDurationText(3900), '1時間5分');
+  assert.equal(routeDurationText(7200), '2時間');
 });
 
 test('ramp and lane wording stays localized after choosing the maneuver form', (t) => {
@@ -892,6 +899,12 @@ test('ramp and lane wording stays localized after choosing the maneuver form', (
       'Toma la salida bruscamente a la derecha',
       'Toma el carril izquierdo',
     ],
+    [
+      'ja',
+      'ランプを斜め左に進んでI-95に入る',
+      '出口を急角度で右に出る',
+      '左車線を通行',
+    ],
     [
       'zh-CN',
       '向左前方驶入匝道，进入 I-95',