Build each screen in one language: the screen texts from the translations, European letters in the fonts, and Language & region in the app

The firmware takes its texts from screen_manager/translations/<code>.json at build time (components/smart_display:
one table in main.cpp, keys in screen_text_keys.h), and the fonts carry every letter European languages write with.
The app keeps Settings -> Language & region (language, clock, numbers) for every screen, writes LANGUAGE into the
screens' YAML and sends the clock and the number format in the layout message; the per-screen 24-hour clock goes.

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

components/smart_display/__init__.py

@@ -1,3 +1,38 @@
-"""Runtime tile configuration transported through the normal ESPHome API."""
+"""Runtime tile configuration transported through the normal ESPHome API.
+
+`smart_display: language: nl` (app 0.2.90) builds the screen's texts in that language: the `screen` section of
+screen_manager/translations/nl.json, with English for any key it lacks, becomes one table in main.cpp
+(screen_text_gen.py, screen_text.h). The shared core sets it from the LANGUAGE substitution, which ESP Screens writes
+into a screen's YAML.
+"""
+import logging
+
+import esphome.codegen as cg
+import esphome.config_validation as cv
+
+from . import screen_text_gen
+
 CODEOWNERS = []
 AUTO_LOAD = ["json"]
+_LOGGER = logging.getLogger(__name__)
+
+CONF_LANGUAGE = "language"
+
+
+def _language(value):
+    value = cv.string_strict(value).strip()
+    if not screen_text_gen.LANGUAGE_CODE.fullmatch(value):
+        raise cv.Invalid("Expected a language code such as en, nl or pt-BR")
+    return value
+
+
+CONFIG_SCHEMA = cv.Schema({cv.Optional(CONF_LANGUAGE, default="en"): _language})
+
+
+async def to_code(config):
+    wanted = config[CONF_LANGUAGE]
+    language, code = screen_text_gen.definitions(wanted)
+    if language != wanted:
+        # ESP Screens only writes languages it has; a hand-written one it lacks builds in the nearest it does have.
+        _LOGGER.warning("No translation for %s yet; the screen's texts are in %s", wanted, language)
+    cg.add_global(cg.RawStatement(code))