It looks to me like the German translation was intented to be informal. That is what most communities would use anyway and what should be the default. So I would consider the formal language in the recent translation a bug.
I also think the translation could be improved. Most parts are very well translated other parts are not so perfect and a few are just wrong.
I'm also not sure, if there is really a need for a formal German translation. It's just a UI. We wouldn't talk to Siri, Google or Alexa in a formal way.
Of course there is nothing wrong to have a formal translation as an option, but would there be a maintainer? Who wants to do the extra work and keep the two translations in sync.
I opened an issue: https://github.com/NodeBB/NodeBB/issues/5619streaps created this issue in NodeBB/NodeBB closed German translation inconsistencies / should we add de-x-formal? #5619