Hi, this plugin uses the Web Speech API for TTS, which uses the local TTS voice from the user's computer. If the user's default TTS voice is in their language it should speak using their language.
For English users, this will most likely be Microsoft David, but for example for Spanish (Mexico) users this may be Microsoft Sabina. You can test out the TTS using a codepen here: https://codepen.io/matt-west/pen/DpmMgE .
I don't believe it would be possible to assign actors to various TTS voices as players would likely have different voices installed so it would not be very good quality as everyone would be hearing different voices. It is not really meant as some voice actor replacement for your game, it is mostly an accessibility feature for people who have difficulty reading commands or other text in your game. However, you could use it for gameplay if you have a need for the standard TTS robotic voice, such as a talking computer system or maybe a robot character in your game.