This is what the press release says about the translation app released with the video:
In response to having a large English-speaking fan base, Yelle has announced a translator app to accompany the lyric video for their single “Complètement Fou.” The lyric video features the French lyrics of the song with the English translation overlapping, creating a complex image. The app works to decode the video by letting the user choose whether they’d like to view the original French lyrics or the English translation by holding their phone up to the music video screen and selecting a language. The user is also able to toggle between the languages in real time.
Video created by Pomp & Clout.