Hi I have got Samsung Galaxy S4 Active.
I have downloaded Samsung's high quality British English TTS engine and set it as "Preferred TTS engine" From Settings -> MyDevice -> Accessibility -> Text-to-speech options -> Samsung text-to-speech engine -> Languages
However when I use Google's Navigation app, it still uses Google's TTS engine.
Furthermore I actually disabled google's TTS engine so it is not present at TTS options, the problem still persists!
The only thing that I have discovered is if I change preferred language from English UK to English US, then TTS engine changes in Google's Navigation App from Google's English UK to English US!
I thought the problem is at Samsung's TTS Engine. So I downloaded Ivona TTS Engine and tried same languages and experiencing same problems with that too.
Is there a solution for this stubborn Google's Navigation app's voice?