Hi Matthias,
Yes copy the de.js instead of it.js
It is the same problem for other languages. I fixed it in version 2.3.9.9 by copying most common language files from the langs folder into themes/advanced/langs. I think the TinyMCE team introduced this bug while consolidating changes for their new 4.x branch but still sharing some code with the 3.x branch. I've been wanting to update mojoPortal to use their newer 4.x branch but they don't have clear documentation yet on how to integrate a custom file manager and I have not been able to figure out how to do it yet.
After all the generous beers you have bought me in the last few years I always try to provide good service whenever you have a question or need help, you are a kind soul and a valued customer!
Best,
Joe