Localize runtime error fallback notifications

[Pigbibi]

Summary

• localize route-level runtime error fallback notifications when NOTIFY_LANG is Chinese
• keep English fallback copy for default/non-Chinese locale
• add request handling coverage for Chinese fallback text

Tests

• python -m py_compile main.py tests/test_request_handling.py
• python -m pytest tests/test_request_handling.py -q (skipped locally: flask is not installed in this VPS environment)