Update messages_de_DE.properties

[mjbohn]

Some minor changes/fixes to german translation.

Description

fixed two minor translation gliches

Checklist

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have attached images of the change if it is UI based
• I have commented my code, particularly in hard-to-understand areas
• If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• My changes generate no new warnings
• I have read the section Add New Translation Tags (for new translation tags only)

[github-actions]

🚀 Translation Verification Summary

🔄 Reference Branch: main-branch

📃 File Check: messages_de_DE.properties

1. Test Status: ✅ Passed
2. Test Status: ✅ Passed

---

✅ Overall Check Status: Success

Thanks @mjbohn for your help in keeping the translations up to date.