All the files you look for are in the source code. Menus and labels are translated in files suffixed by _de.properties (the biggest part being in the src/com/eteks/sweethome3d/swing/package_de.properties file) and help files are HTML files available in src/com/eteks/sweethome3d/viewcontroller/resources/help/de directory.
I know that German translation isn't complete. If you want to help finish the translation, please translate the pages drawingRooms.html, editingRooms.html and addingTexts.html.