Either this is a well kept secret of I’ve been asleep, but I’ve only just found Microsoft provide easy access to the SAPI voices provided with other language versions of Windows. The following is the case in my Windows 8.1 Pro but your mileage may vary as they say.
There don’t appear to be voices for every language, for example while I got German, I didn’t get one for Greek. There are several provided for Spanish.
In order to get the extra voices you simple go to Control->Panel->Language and add a language. When you double click on the new language you get (after a pause) an option to download and install a ‘Language pack’. This install takes forever, but eventually the new SAPI voices then appear in the Windows TTS options as usual.
Additionally the language at the top of the list selects the system used for Windows text and is also reported to programs. You will need to log out or reboot for a change to take effect. We use this in the latest Maavis installer to provide internationalisation support for the Cloud4All project.
Microsoft have done an excellent job with internationalisation – it’s a very clean user experience.