Voice Commands function allows you to partially use the app with just your voice. You need to enable it in Main menu > Settings > Voice Commands first.
To use the preset voice commands, simply move or place your hand very close to the top of the front part of your phone (the proximity sensors are usually at the top of the phone). The voice commands screen will appear, showing you the available options for voice commands.
The language shown in the list of voice commands depends on the regional settings of the device, not on the language chosen in Sygic GPS Navigation.