The solution itself was quite simple, but it took me a while to find it. If you compile the .Net project Visual Studio creates a "en"-Folder in your output folder (e. g.: bin/Debug/). This folder is not created with the compilation of the mixed mode version, so just copy it from your .Net output folder to your mixed mode output folder. In addition I'll create a little script to copy it to the needed location after the build.
Have a good one everybody.