My NodeBB instance runs on Alpine Linux 3.5.2 inside a SmartOS lx zone...
Debian 7 is quite old. Try something more recent.
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/5619