How does Google TTS decide which voice to use?

I have never seen Google TTS switch languages based on the text it is trying to "read", the default language is based on the system language, meaning if you select English as the display language, English is the TTS language by default but you can change it in the Language and Keyboard settings. Are you asking if an ebook is written in English and Spanish (for example) you want the TTS engine to read each portion in it's written language? I don't think that is currently possible for the Android TTS engine to have an "auto detect" setting.

You could request this feature, see

productforums.google.com/forum/#!home and poke around, I am not sure it has it's own section... all Google products are supported in their forums.