The song "Je vole" was written by Michel Sardou as a kind of suicide note of a teenager [as he explained in his autobiography].
In the movie the lyrics were slightly changed (except the refrain) to fit to the theme of the movie, which is leaving home for a career, not suicide.
The actors quickly discovered that it's extremely hard to sign and speak at the same time. This proved to be particularly grueling for Louane Emera in the film's memorable climax where she has to sing and sign simultaneously.
Luca Gelberg (Quentin Belier) is, in fact a deaf person.
The actors spent roughly 4 months learning sign language.
Michel Sardou (1947-) is a French singer and songwriter. He is known for his love songs such as "La Maladie d'Amour", "Je vais t'aimer" (both used in the film) as well as for songs dealing with social and political issues. His popularity has spanned 50 years, as of 2015.