fix german translation

[richardtreier]

The german translation is out-of-sync with the english properties:

• The original PR was alive too long.
• We do not want to translate all technical terms as in the original translation.
• Keys are out of sync, so the entire german translation has to be largely redone
• Let's get some help from the Biz team for this, but we need to technically prepare it.

All strings need to have been externalized for this (other issue: #828)

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1. fix english I18n strings & externalize all strings #828
   kind/enhancement +1
   
2. When keys are collected, the german strings can be re-translated. Creating an empty file of "key": "key" can help a non-technical person translating the strings while checking the UI