You should look at System.Speech.Recognition.SpeechRecognitionEngine - it's an 'in-proc' recognizer that will let you specify the language you want.
Your next problem is that en-US Vista doesn't ship with the spanish recognition engine. For that, you'll need the Spanish Language Pack. Once you install that, you should be able to instantiate a spanish recognition engine like this:
using System.Speech.Recognition;
SpeechRecognitionEngine recognizer = new SpeechRecognitionEngine(new CultureInfo("es-ES"));
At that point, you can install grammars & do recognitions, etc.