Translating a text for a bilingual book is not as simple as translating a text into another language. In the latter case, there is space (within certain limits) for so-called "reinterpretations" where terms, expressions, and even entire sentences are non-translatable, or at least their literal translation would appear to be a plain stretching of the language. In these cases, the linguistic and literary artistry of the translator can be called upon.
In bilingual books, however, too many reinterpretations can prove counteractive, due to their "mirror" nature, and especially if the reader is still familiarising themselves with the language. We could argue that not all texts are suitable to becoming bilingual, although we cannot have bilinguals without some reinterpretation. What appears grammatically correct is not always the best translation and therefore does not effectively help people to learn a language in its natural form. A translation "sounds" natural when it is saying exactly the same thing as the original but in its own manner, and in bilinguals these differences can become a great way to familiarise oneself with the language in an effective way.
For example: in Cos'hai nella pancia/Let It Out, the English version of this title is not a literal translation of the original Italian title. In reality, a grammatically correct English version does exist: "What Have You Got in Your Belly?"; however, this version didn't "sound" quite right to us, and we decided to change it with a more "catchy" one, as often happens in translations. Most languages have different ways of expressing similar concepts. In a bilingual, the literal difference is usually obvious, but hopefully this encourages learners to appreciate the sense of the intended meaning, while familiarising themselves with a different ("foreign") means of expression.