English language support (i18n)

[robert197]

Description

Add English language support alongside German. Django i18n framework.

Requirements

• Django i18n/l10n framework setup
• All user-facing strings wrapped in gettext
• German (.po) and English (.po) translation files
• Language switcher in UI
• URL prefix: /de/ and /en/

Acceptance Criteria

• All pages available in German and English
• Language switcher works
• Email templates in user's preferred language