There are ~50 locales/languages one can load (additional) on Windows (at least Windows Pro). When adding, you have a checkbox choice. You can select all the locales/languages and see how many are then available to Mathematica after loading. And How many are useful. I indicated a solution where you only chose German.
One question was
"Any ideas how to particularly install the German voices would be
greatly appreciated."
So you don't have to do this each time. You could have chosen more items to install more in one go.
On a Windows system I believe Mathematica may be utilizing Speech Application Programming Interface (SAPI) 4 and 5. I don't know what the equivalent is on Mac OS or Nix variants. But there are some Text-To-Speech TTS standards and file formats. The files that create the rules of how sound is produced from text etc.
There are free Text to Speech Natural Voices (SAPI 4 and SAPI 5). I have not tried them nor can vouch. But that would be another way to add Voices to your system.
But I believe when the VoiceStyleData[] examples was created and on whose ever system, that is what they had ~52. As Hans Milton posted if you re-run the examples you get something different based on your system.
