Envoyé par MSDN
I understand; the available voices (TTS) by default in Windows 7 are the ones for your UI language pack; the voices provided are tightly binded to the language. I mean, if your UI language is english, the voices are only english. This is valid for the default voices that come with Windows 7.
There are third parties that offers multilanguage TTS, enabling changing them on the fly. I never tried them, but looks like you can change the voice using System.Speech.Synthesis.SpeechSynthesizer.SelectVoiceByHints and passing the culture you want.