Dionysos is an opera by Wolfgang Rihm based on Friedrich Nietzsche's "Dionysian-Dithyrambs". The composer wrote the libretto and subtitled his work: "Opernphantasie nach Texten von Friedrich Nietzsche / Szenen und Dithyramben" ("Operatic fantasy after texts by Friedrich Nietzsche / Scenes and dithyrambs). It premiered at the Salzburg Festival on 27 July 2010.
The opera was written in German
The context states that the opera was given a German name, and then has the English translation in brackets. So therefore we can assume the opera was written in German. The system did likely not comprehend that because the title of the opera was in German it means the work was German. 
( z1 write :ARG1 ( z2 opera ) :ARG3 ( z3 language :name ( z4 name :op "German" ) ) )