The german language files for Ground Control.
Just copy the files from the archive into the game dir (e.g. ‘Steam\steamapps\common\Ground Control Anthology’).
The language files are taken from the german retail version – I’ve tested them with the ‘Ground Control – Anthology’ version avaible on Steam. It works, but some parts of the menu/interface and the whole ‘Dark Conspiracy’ expansion are still in english. All cutscenes, mission briefings and voices of the main campaign are fully in german.
With the english retail version itself there should be no problems after all (but I’ve never tested it).