Hi!
I guess I found the reason why the clock didn't' work. And I think this might be helpful for others having the same issue.
The problem is the CPU speed of the PI. As it is noticed in the DGTPi Instructions.txt you have to set the core_freq and core_freq_min in the config.txt to slow down the PI for being able to communicate with the clock. I experimented with some values and found out that for my DGT3000 Limited Edition both values set to 300 do the job. The clock is working fine. (It used to be 500, but this doesn't work). Maybe different clocks need other values?!
But the slow speed of the voice announcing the moves remains. Sometimes it takes about 5 sec from the flashing of the Chessnut's LEDs till the voice starts announcing, sometimes some syllables are cut off.
One little bug: changing the language doesn't effect the speakers language, the picochess.ini still has en:christina. I had to change this in the ini-file.