In this article, you'll learn how to change the voice that Mate for iOS uses for its speak-out feature. You can choose between different accents and female or male voices.
Actually, Mate is using voices that are built in iOS, so to change the voice for a specific language, you'll have to tweak iOS settings.
To do so, open iPhone/iPad's Settings and go to Accessibility > VoiceOver > Speech. Then you can tap Add New Language to add a language you want a new voice for. Tap on the newly-added language to select a voice for it. Note that non-default languages (they're often better) require a manual download before you can set them.