1. Make sure you have installed TTS engine:
https://play.google.com/store/apps/details?id=com.google.android.tts&hl=en[
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2. Set the language you want to use:
http://developer.android.com/reference/android/speech/tts/TextToSpeech.html#setLanguage(java.util.Locale)[
^] For example:
mTts.isLanguageAvailable(Locale.UK))
mTts.isLanguageAvailable(Locale.FRANCE))
mTts.isLanguageAvailable(new Locale("spa", "ESP")))
mTts.setLanguage(Locale.US);
String myText1 = "Did you sleep well?";
String myText2 = "I hope so, because it's time to wake up.";
mTts.speak(myText1, TextToSpeech.QUEUE_FLUSH, null);
mTts.speak(myText2, TextToSpeech.QUEUE_ADD, null);
For more information, you can read
An introduction to Text-To-Speech in Android[
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