In Windows 10 or 11 go to Settings and search for Voice. Really looking for the Speech settings.

Choose "+ Add voices" from "Manage voices" Load the German (Germany) voice package

After loading voice files or files, and from a fresh Mathematica kernel run (not install)

VoiceStyleData[] // Dataset

You should get at least for German (Germany)

Female German Hedda

added to list

The default for VoiceStyleData[] is Automatic which is what your system supports

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.

This is exactly the expression I evaluated on a newly rebooted machine, the only difference is I told it to display as a table using the //Dataset function. No previous lines are executed. My version is 14.1.0.0, Windows version. Nothing else was modified from a "normal" install.