Change the voice for speech output
The speech output in the Quiz by default uses the default system voice for the selected language. macOS and iOS come with pre-installed voices for all languages.
However, in the system settings you can install high definition versions of those voices and also download different voices. For some languages you can also install different local accents. For example you can install voices for Australian, Canadian or South African English (among others).
To do that, go to System Settings > Accessibility > Speech > Voices
Technically, Wokabulary unfortunately cannot determine which voice you set as default for each language. Instead we always use those voices where an Enhanced Quality version is available. So if you want Wokabulary to use a specific voice, make sure to only have one enhanced quality version installed for each language.