Notation like Home Assistant, each screen in its own language, and the language in every build

From the end-to-end test on the bench boards and an independent review:
- Units spaced as Home Assistant does (54% / 54 % in German and French, 18°, 21.5 °C), in the firmware and the app.
- Grouping from five digits where the language does (1234 but 12.345 in Spanish, Polish, Portuguese): the layout
  message carries group_min and percent_space; an unknown number style no longer refuses a layout.
- Automatic follows Home Assistant's own language for the clock and numbers, also without a translation (Swedish:
  24 hours, 1 234,5; pt-BR: 1.234,5): regions.json from the CLDR for all 63 languages Home Assistant offers.
- 12-hour clock: the top bar says 3:48 PM, the clock card 3:48 (not 03:48), Last runs follow the clock.
- The climate setpoint font has the decimal comma; the value fonts have ², ³ and µ (m³ was a box).
- Weather days: at most three letters without the dot (mié, sáb, czw); the screen took three bytes.
- Texts fall back per key through the base language (pt-BR -> pt -> en) in the firmware and the app.
- Each screen gets the app's words in the language its firmware speaks; firmware from before the languages gets
  English and its own letters, not boxes.
- set_language re-reads what it writes (indentation, comments, quoted keys, CRLF, !include) and every build writes
  the language first; the 24-hour pin no longer hits a fresh install and waits for the old clock switches.
- Tests: region and notation, plural parity with the C++ rules, set_language edge cases, the pin, PUT /api/language.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

screen_manager/app/core.py

@@ -1533,6 +1533,9 @@ def discover_screens(registry, states, devices, areas):
     nodes = diagnostic(NAME_DEVICE_NAME, r'[a-z0-9][a-z0-9-]{0,30}')
     addresses = diagnostic(NAME_IP_ADDRESS, r'\d{1,3}(\.\d{1,3}){3}')
     languages = diagnostic(NAME_SCREEN_LANGUAGE, r'[a-z]{2,3}(-[A-Za-z0-9]{2,8})?')
+    # Firmware from before the languages (0.2.75 and older) has no such sensor: it speaks English, with fewer letters.
+    speaks = {item.get('device_id') for item in registry
+              if item.get('platform') == 'esphome' and item.get('original_name') in NAME_SCREEN_LANGUAGE}
     screens = []
     for item in registry:
         eid = item['entity_id']
@@ -1551,6 +1554,7 @@ def discover_screens(registry, states, devices, areas):
                         'board': boards.get(item.get('device_id'), 'unknown'),
                         'node': nodes.get(item.get('device_id')), 'ip': addresses.get(item.get('device_id')),
                         'language': languages.get(item.get('device_id')),
+                        'language_sensor': item.get('device_id') in speaks,
                         'device': device.get('name') or '',
                         'area': area, 'online': state.get('state') not in (None, 'unknown', 'unavailable'),
                         # What the screen reports (a word of the firmware's protocol), or ours in English, which the editor