Reworked implementation of the multilingual-response feature (issue #218)
as a reusable, framework-level capability instead of logic bolted onto
HelloCommand.
- modules/lang_detector.py: keyword-first detection (reliable for the short
greetings typical on mesh) with optional langdetect for longer text;
results are constrained to languages that actually have a translation file.
- core.py: cache Translator instances by language (get_translator) so
per-message switching never re-reads files, and expose available_languages()
derived from translations/*.json. Store translation_path; keep cache/path
consistent on config reload.
- base_command.py: task-local translator override via a ContextVar (safe under
asyncio concurrency, no global mutation), plus detect_response_language() and
a respond_in_sender_language() context manager any command can opt into.
- hello_command.py: build the reply inside respond_in_sender_language(); the
only awaited call (send_response) stays outside the override window.
- Gated by [Localization] auto_detect_language (default false); langdetect is
an optional [lang] extra, not a hard dependency.
Tests: unit coverage for the detector and the base-command wiring.
