Improvement suggestion
The notification Prêt impossible car l’exemplaire est déjà prêté in the circulation interface should be replaced by Prêt impossible car l’exemplaire est déjà prêté à un autre lecteur.
The text is ambiguous: when the librarian try to loan the item to patron A and receive this warning, he can think the item is already on loan for patron A. But in fact it's already on loan for patron B.