Add internationalization support with translation data for English and
German languages. This provides the foundation for localized formatting
of dates and durations.
Changes:
- Add i18n.go with translation maps for month names, weekday names,
and duration units (both EN and DE)
- Implement helper methods on Lang type: MonthName, MonthNameShort,
WeekdayName, WeekdayNameShort, DurationUnit
- Add automatic fallback to English for unknown languages
- Create comprehensive test suite in i18n_test.go (97.8% coverage)
- Update Lang type documentation with future expansion notes
- Add example tests demonstrating i18n usage
Blocks: quando-10t, quando-5ol, quando-95w
