V2: Translation system refactor

[Nielsvanpach]

Summary

• Remove HasTranslations interface and Translatable trait — translation is now built into the Country base class
• Translations are loaded once per get() call instead of once per holiday name (N file reads → 1)
• Translation file paths changed from lang/{hyphenated-class-name}/ to lang/{countryCode}/ (e.g. lang/germany/ → lang/de/)
• Any country can now have translations by just adding a JSON file — no code changes needed

Breaking changes

• HasTranslations interface removed
• Translatable trait removed
• defaultLocale() is now a protected method on Country (was public on interface)
• Translation file paths changed