You can try this it works for me.
public VoiceInfo _defaultVoice;
//*********************************************
// TTS VOICE
// 0 - Javier - male spanish
// 1 - Jill - female english
// 2 - Paulina - female spanish
// 3 - Tom - male english
//*********************************************
this.TelephonySession.Synthesizer.DefaultVoice = this.TelephonySession.Synthesizer.InstalledVoices[2];
// Note the original VoiceInfo.
_defaultVoice = this.Synthesizer.DefaultVoice;
// set spanish TTS
Thread.CurrentThread.CurrentCulture = new CultureInfo("es-MX");
this.main.TelephonySession.CurrentUICulture = Thread.CurrentThread.CurrentCulture;
askSpanishMenu.MainPrompt.AppendText("Para repetir estas opciones oprima la estrella. Para regresar a las opciones anteriores oprima el numeral.");
Then to change back to english as default, if you are using mixed language apps.
Thread
.CurrentThread.CurrentCulture = new CultureInfo("en-US");
this.main.TelephonySession.CurrentUICulture = Thread.CurrentThread.CurrentCulture;