feat(quando-zbr): implement i18n infrastructure for EN/DE languages

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

i18n.go

@@ -0,0 +1,138 @@
+package quando
+
+import "time"
+
+// i18n.go - Internationalization support for quando
+//
+// This file contains translations for month names, weekday names, and
+// duration units used in formatting operations.
+//
+// Phase 1 Languages:
+//   - EN (English) - Default
+//   - DE (Deutsch/German)
+//
+// Future expansion will add 21 more languages including:
+//   FR, ES, IT, PT, NL, PL, RU, JA, ZH, KO, AR, HI, TR, SV, NO, DA, FI, CS, HU, RO, UK, EL
+//
+// i18n applies to:
+//   - Format(Long): "February 9, 2026" vs "9. Februar 2026"
+//   - FormatLayout with month/weekday names
+//   - Duration.Human(): "10 months, 16 days" vs "10 Monate, 16 Tage"
+//
+// i18n does NOT apply to:
+//   - ISO, EU, US, RFC2822 formats (always language-independent)
+//   - Numeric outputs (WeekNumber, Quarter, DayOfYear)
+
+// monthNames contains full month name translations.
+var monthNames = map[Lang][12]string{
+	EN: {
+		"January", "February", "March", "April", "May", "June",
+		"July", "August", "September", "October", "November", "December",
+	},
+	DE: {
+		"Januar", "Februar", "März", "April", "Mai", "Juni",
+		"Juli", "August", "September", "Oktober", "November", "Dezember",
+	},
+}
+
+// monthNamesShort contains short (3-letter) month name translations.
+var monthNamesShort = map[Lang][12]string{
+	EN: {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"},
+	DE: {"Jan", "Feb", "Mär", "Apr", "Mai", "Jun", "Jul", "Aug", "Sep", "Okt", "Nov", "Dez"},
+}
+
+// weekdayNames contains full weekday name translations.
+// Index: Sunday = 0, Monday = 1, ..., Saturday = 6
+var weekdayNames = map[Lang][7]string{
+	EN: {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"},
+	DE: {"Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag"},
+}
+
+// weekdayNamesShort contains short (3-letter) weekday name translations.
+// Index: Sunday = 0, Monday = 1, ..., Saturday = 6
+var weekdayNamesShort = map[Lang][7]string{
+	EN: {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"},
+	DE: {"So", "Mo", "Di", "Mi", "Do", "Fr", "Sa"},
+}
+
+// durationUnits contains duration unit translations for Human() formatting.
+// Each unit has singular and plural forms: [0] = singular, [1] = plural
+var durationUnits = map[Lang]map[string][2]string{
+	EN: {
+		"year":   {"year", "years"},
+		"month":  {"month", "months"},
+		"week":   {"week", "weeks"},
+		"day":    {"day", "days"},
+		"hour":   {"hour", "hours"},
+		"minute": {"minute", "minutes"},
+		"second": {"second", "seconds"},
+	},
+	DE: {
+		"year":   {"Jahr", "Jahre"},
+		"month":  {"Monat", "Monate"},
+		"week":   {"Woche", "Wochen"},
+		"day":    {"Tag", "Tage"},
+		"hour":   {"Stunde", "Stunden"},
+		"minute": {"Minute", "Minuten"},
+		"second": {"Sekunde", "Sekunden"},
+	},
+}
+
+// MonthName returns the localized month name for the given language.
+// Returns English name if language not found.
+func (l Lang) MonthName(month time.Month) string {
+	if names, ok := monthNames[l]; ok {
+		return names[month-1]
+	}
+	// Fallback to English
+	return monthNames[EN][month-1]
+}
+
+// MonthNameShort returns the short (3-letter) localized month name.
+// Returns English abbreviation if language not found.
+func (l Lang) MonthNameShort(month time.Month) string {
+	if names, ok := monthNamesShort[l]; ok {
+		return names[month-1]
+	}
+	return monthNamesShort[EN][month-1]
+}
+
+// WeekdayName returns the localized weekday name for the given language.
+// Returns English name if language not found.
+func (l Lang) WeekdayName(weekday time.Weekday) string {
+	if names, ok := weekdayNames[l]; ok {
+		return names[weekday]
+	}
+	return weekdayNames[EN][weekday]
+}
+
+// WeekdayNameShort returns the short (3-letter) localized weekday name.
+// Returns English abbreviation if language not found.
+func (l Lang) WeekdayNameShort(weekday time.Weekday) string {
+	if names, ok := weekdayNamesShort[l]; ok {
+		return names[weekday]
+	}
+	return weekdayNamesShort[EN][weekday]
+}
+
+// DurationUnit returns the localized duration unit name (singular or plural).
+// The plural parameter determines which form to use.
+// Returns English name if language not found.
+func (l Lang) DurationUnit(unit string, plural bool) string {
+	if units, ok := durationUnits[l]; ok {
+		if forms, ok := units[unit]; ok {
+			if plural {
+				return forms[1]
+			}
+			return forms[0]
+		}
+	}
+	// Fallback to English
+	if forms, ok := durationUnits[EN][unit]; ok {
+		if plural {
+			return forms[1]
+		}
+		return forms[0]
+	}
+	return unit
+}