I and some friends are trying to translate the game to PT-BR, and so we need the files containing the texts of the game (UI, Books, Quests, etc), can someone tell me where to find these files?

They don't exist. They can be generated by the CK but not until it's released.

Just about all of the texts are contained in the files in the Data\Strings directory. There is no editor yet for those, so it'd be easiest to wait for the CK.