I've asked via PM I've asked in the update thread and I've reminded them day after day.
GOG has confirmed to me that they have received the update for Cognition including the german subtitles for all episodes TWO WEEKS ago and it should only take "a few days" until it is available.

THE UPDATE STILL IS NOT AVAILABLE. And all my requests that they please tell me an estimated update date are being ignored. I am getting really pi**ed and so I ask here again.

WHEN WILL THIS GAME FINALLY BE UPDATED TO THE ACTUAL VERSION WHICH GOG ALREADY HAVE?

GOG now has told me that the files they have received from the developer don't function like they should do. This is the only reason for the delay. They already contacted the developer and they will update the game as soon as the technical issues have been sorted out.

So I guess this thread can be closed (and now that I know this I am feeling a bit silly to have created it at all ;)).

Newsflash: Cognition has just received German Subtitles for the Windows version - Mac is coming later. :)