The libretto of Tristan und Isolde and Ariane et Barbe-bleue are not in the same language.
Tristan und Isolde ("Tristan and Isolde", or "Tristan and Isolda", or "Tristran and Ysolt") is an opera, or music drama, in three acts by Richard Wagner to a German libretto by the composer, based largely on the romance by Gottfried von Strassburg. 
 The French libretto is adapted (with very few changes) from the symbolist play of the same name by Maurice Maeterlinck, itself loosely based on the French literary tale "La Barbe bleue" by Charles Perrault. 
