It has now been posted in the PHOENIX forum that the German subtitles will be included in the digital versions of the game soon and this hopefullx includes the GoG version. They will do it after they have released GK and fixed critical bugs. http://www.postudios.com/blog/forum/index.php?topic=13143.msg360064#msg360064
"Sorry for the long pause between replies--it IS coming. Right now, as you can imagine, we've been hyper focused on GK. Once that's released, we'll be able to get back to a number of things that we've been wanting to finish up and release, like the German Cog translation. :)"