Add internationalization support for English and Simplified Chinese

[xiaoming728]

• Add English as the fallback locale.
  • Add Simplified Chinese (zh-CN).
  • Detect the current Obsidian locale automatically.
  • Translate plugin-owned UI text, notices, commands, settings, dialogs, and date labels.
  • Keep user-defined project data unchanged.
  • Add tests for locale detection, fallback, interpolation, and date messages.
  • Do not add a manual language selector in the first iteration.