are Boulevard Solitude and Moses und Aron both German librettos?
Boulevard Solitude: Boulevard Solitude is a "Lyrisches Drama " (lyric drama) or opera in one act by Hans Werner Henze to a German libretto by Grete Weil after the play by Walter Jockisch, in its turn a modern retelling of François Prévost's "Manon Lescaut". Moses und Aron:  The German libretto is by the composer after the Book of Exodus.
yes