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.
Open System Preferences > Accessibility > Speech > Text to Speech.
Click on the System Voice: menu and select Customise.
You're presented with a huge list of voices to choose from. Click on a voice and press Play to preview it. Check the boxes to the left of the ones you like. Then click OK.