It was composed to a libretto in the French language by Camille du Locle and Joseph MÃ ry . It was based on a famous German play called Don Carlos , Infante of Spain by Friedrich Schiller . Don Carlos is a five-act Grand Opera composed by Giuseppe Verdi to a French language libretto by Camille du Locle and Joseph MÃ ry , based on the dramatic play Don Carlos , Infant von Spanien ( `` Don Carlos , Infante of Spain '' ) by Friedrich Schiller .