Each child has a language they tell stories in and one or more languages they are learning, with one main one. New books are written in the main learning language.
To read a book in another of their languages, switch language in the reader: every sentence has a reading in each of your child's learning languages, with its own voice and highlighting. Translation is always free.
Adding a language
Open your child's settings and add a learning language. Every pair works: tell in one language, read in another.