seeing that the german patch for Thief Gold doesn't work (at least its not supporting Windows 8.1), i will create an archive to overwrite all the sound and video files, but its around 530MB and will take some hours until its uploaded because i have really slow upload speed.
edit: here it is: https://docs.google.com/file/d/0B7-OScs6HK4hWmJOQ0xBc1M0MFU/edit
its a .rar archive, extract it and copy the 2 folders (MOVIES & RES) into your thief gold main directory. overwrite all files. open the install.cfg in the thief gold directory and change the line where it says "language english" to "language german".
that should do it. be sure to install TGfix before or after, i encountered some menu anomalies without it.
i will do one for thief 2 in the next few days.