You require an internet connection for the Text to Speech API, but you can use Text to Speech in offline mode by applying the following settings.
Currently not all devices support offline speech input. However you can follow this discussion to enable offline speech input for supported devices.
1. On your device go to Settings -> Language and Input. Click on the Google voice input icon (text-to-speech output on some devices).
2. Under ALL tab select the language you want to download.
3. Once the language package downloaded, you can see it under INSTALLED tab.
Download all the offline speech models (voices) you require to support offline Text to Speech.
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