The languages available to you in Read to Me are dependent on which languages are installed on your operating system.
Changing the language
Click on the Settings cog and then choose the language you want (note, you can choose various accent/dialogues from within a language). The example below is using the Chrome browser on MacOS 10.15.4.
How do I add more languages?
Book Creator will only show the languages you have installed on your computer, so you'll need to follow the instructions for adding languages to your computer.
Chromebook (last two sections)
Open System Preferences > Accessibility > Spoken Content > System Voice
Click on the System Voice dropdown menu and select Customise.
You're presented with a list of voices to choose from. Click on a voice and press Play to preview it. If you'd like to explore more voices then click on 'Customise' and you'll be able to scroll through the voices available.