code: public final ListVoicesResponse listVoices(String languageCode) {
  
      ListVoicesRequest request =
          ListVoicesRequest.newBuilder().setLanguageCode(languageCode).build();
      return listVoices(request);
    }
docs: Returns a list of [Voice][google.cloud.texttospeech.v1beta1.Voice] supported for synthesis.
  
  <p>Sample code:
  
  <pre><code>
  try (TextToSpeechClient textToSpeechClient = TextToSpeechClient.create()) {
  String languageCode = "";
  ListVoicesResponse response = textToSpeechClient.listVoices(languageCode);
  }
  </code></pre>
  
  @param languageCode Optional (but recommended)
  [BCP-47](https://www.rfc-editor.org/rfc/bcp/bcp47.txt) language tag. If specified, the
  ListVoices call will only return voices that can be used to synthesize this language_code.
  E.g. when specifying "en-NZ", you will get supported "en-&#42;" voices; when specifying
  "no", you will get supported "no-&#42;" (Norwegian) and "nb-&#42;" (Norwegian Bokmal)
  voices; specifying "zh" will also get supported "cmn-&#42;" voices; specifying "zh-hk" will
  also get supported "yue-&#42;" voices.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails