Die Zauberflöte
In the opera house scene, the opera being performed is Die Zauberflöte, an opera whose libretto is in German, and it is being performed in Vienna, a German-speaking city. But it is not being performed in German; it is a little hard to make out but it sounds like Italian. I rather doubt a performance of the opera in Italian, or any language other than German, has ever taken place in Vienna. Did this curious choice in production conceivably stem from a dislike of Ophüls' for the German language after his experiences of the 30s and beyond?
The aria heard (Ein Mädchen oder Weibchen) is somewhat ironic in the context of the film: http://www.aria-database.com/translations/mflute20_madchen.txt
"I beseech ye in the bowels of Christ, think that ye may be mistaken."