Consider the type of book you are creating. Is it a children's picture book with a few words in another language, or is it a complete, word-by-word translation of a complex, award-winning book?
Decide whether you want both languages on the same page, on facing pages or if one language should be in the notes or at the back of the book. Children's picture books, for example, use both languages together, frequently on the same page. Translated works, such as Stephen Mitchell's translation of Pablo Neruda's poetry in "Full Woman, Fleshly Apple, Hot Moon," usually appear with the languages on opposing pages so that readers can compare the use of terms and metaphors. Educational treatises usually contain reference notes at the bottom of pages under the pertinent information. Some people use a line written in one language, with the same line written just below in the other language. This method should be done using two different fonts, otherwise it can be confusing.
Try out the different ways of arranging your text (on the same page, on facing pages, one line in one language and the next in the other, and so on) and try each one out until you find the one that best suits the type of book you are creating.
Decide on the style you like the best.