Opera titles in Russian, Czech, and Hungarian tend to be translated into English when performed, but those in Italian, German, and French are given in the original language, with a number of exceptions. Here’s a handy list:

The Barber of Seville
Dialogues of the Carmelites
The Flying Dutchman
Hansel and Gretel
Madam Butterfly
The Magic Flute
The Marriage of Figaro
The Ring of the Nibelungen
Romeo and Juliette
Samson and Delilah
Tristan and Isolde
William Tell