Under settings -> Language & Speech, you can choose your default voice and any other voices you want to switch to. These can be in any language that you can see.
Press the + button in the upper right. Then search or scroll down to find your voice. You can preview the voice first by tapping the small play icon to the right of each name.
There are offline and online voices. Note online voices will only work if you are online. So be aware!
To remove a voice. Swipe right on a voice - but make sure it is not the currently active voice. Once finished, press done, swipe down, or press the x in the upper left.