Пример кода ниже должен помочь вам
var RecogCfg = new Google.Cloud.Speech.V1P1Beta1.RecognitionConfig {
// Encoding = EncodeType,
//SampleRateHertz = Int32.Parse(SampleRate),
LanguageCode = Lang,
// EnableAutomaticPunctuation = true,
//EnableWordTimeOffsets = true,
MaxAlternatives = 5,
Model= "default"
};
//add one or more alternative language codes
RecogCfg.AlternativeLanguageCodes.Add("<Language Code 1>");
RecogCfg.AlternativeLanguageCodes.Add("<Language Code 2>");