We use system text-to-speech engine for voice instructions.
To manage settings for speech synthesis, on your device open Settings → Language and input → Speech → Text to speech output.*
Here you can manage settings for speech synthesis, for example, download additional language pack or select preferred text-to-speech engine (if you have few).
*The exact way to open TTS settings depends on your Android device
If you don't see the settings, make sure you have installed TTS on your device. Many Android devices use Google TTS, you can download it from Google Play. If you disabled TTS engine, enable it first: open device's settings → Apps → Disabled → Select your TTS (for example, Google Text-to-Speech Engine) and press Enable.
For Arabic, Greek and Romanian languages you may need to install additional TTS (as Google TTS doesn’t support the languages yet) from Google Play like Vocalizer TTS or SVOX Classics. They are paid, but you can use free trial.