You will find it useful to check out the web site of the MBROLA speech synthesis project where you will find German diphone databases and a basic German TTS converter. The MBROLA group cooperates with the BLINUX group and others working on utilities for the blind, and have also developed a text input aid based on common Braille abbreviation techniques.
The MBROLA tools are available for most operating systems, under GNU licence conditions or similar.