add code text to speech voices windows 7 ?

Question

I run the code below only the code "en-US, en-GB" I want to add the code "vi-VN" in win7, what should I do ?

 

foreach (RecognizerInfo ri in SpeechRecognitionEngine.InstalledRecognizers())

{

        System.Diagnostics.Debug.WriteLine(ri.Culture.Name);

}