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You can use "languageCode": "es-ES" and "ssmlGender": "FEMALE" properties for Spanish(Female voice).
You can also choose to use a specific voice name such as To be more specific, use "name": "es-ES-Wavenet-B" Spanish(Male) to use standard Standard or Wavenet voices. https://cloud.google.com/text-to-speech/docs/voices
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The language selected in the Deployment screen should be added prior in the Dialogflow agent.
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