Client-side i18n system with localStorage persistence:
- Shared i18n module (frontend/src/client/i18n.ts) with 120+ translation keys
- Language toggle buttons in header on all pages (including login)
- data-i18n attributes on all static translatable elements
- t() function for dynamically rendered content (calculator results, timeline)
- onLangChange callbacks re-render dynamic content on language switch
- Date formatting adapts locale (de-DE / en-GB) per language
- Replaces old dual-display pattern (card-en spans) with single-language switching
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