Add internationalization (I18n)

[dboehmer]

• add translation code
• add texts translated to German
• localize number and date formats
• show errors if input fields have invalid format

[dboehmer]

• sort order including umlauts (ÄÖÜ)

[dboehmer]

Useful resources:

• https://developers.google.com/search/docs/advanced/crawling/managing-multi-regional-sites
• https://developers.google.com/search/docs/advanced/crawling/localized-versions