which LibreCAD version do you use?
The window you are talking about is a Qt standard dialog. The translation for this dialog is not found on the translation server.
For the Qt dialogs there should be a qt_de.qm in the LibreCAD\resources\qm\ folder beside the librecad_de.qm and plugins_de.qm files.
The qt_xx files were added before LibreCAD 2.0.0rc2 version. If your version is older than that, you have to upgrade. When your version is 2.0.0rc2 or above there is another problem. Then you have to check if the qt_de.qm file exists in your installation.
The problem is, that there are languages with the old system, one qt_??.qm file with all translations and couple of languages with the qt_de.qm system.
We have to figure out, how we could handle this in a common way and why this two different solutions exists in Qt.
I have tested this now and it worked for me as expected.
I fixed it with commit d9a59073.
With the next release 2.0.5 installer the Qt library translation files should be installed correct on Windows.
If this is an issue on Debian/Ubuntu Linux, one can add the package qttranslations5-l10n using the preferred packet manager. Then add the translation path /usr/share/qt5/translations in LibreCAD's application settings. Use a semicolon (;) to separate the path from the recources/qm path.
After a restart of LibreCAD the Qt translations should be loaded.
On OSX this is an issue on too, when LibreCAD is build with Qt5. I don't know where to solve this for OSX.