git clone https://github.com/changemyminds/Google-Cloud-TTS-Android.git
Go to here and change "YOUR_API_KEY" to your Google Cloud API Key.
static final String API_KEY = "YOUR_API_KEY";
Note
If you don't know the Google API Key, please see Google document.
Don't forget your API-Key Application restrictions and API restrictions must select None and Don't restrict key.
See the following achievement.
If you want to test voice or find support language, you can go here to test online.
Google Cloud Java Issue
Google Cloud Text-to-speech
OkHttp
Gson