feat: add German language support (de-DE)

[harshanaDisa]

Summary

Related Issues

Changes

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or build changes

Verification

Steps to reproduce / test

What you personally verified

Evidence

• CI passes (pnpm check && pnpm lint && npx tsc --noEmit)
• Manually tested locally
• Screenshots / recordings attached (if UI changes)

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added/updated documentation as needed
• My changes do not introduce new warnings