An early summer visit to Volksoper Wien reaffirmed why it remains one of Europe’s most imaginative lyric theatres. Few companies today show comparable confidence in programming, moving effortlessly from the great operetta tradition of Kálmán, Strauss and Lehár to a problematic Offenbach masterpiece and, most admirably, to Viktor Ullmann’s Der Kaiser von Atlantis. At a time when many houses reduce operetta to seasonal nostalgia or neglect twentieth-century rarities altogether, the Volksoper demonstrates both artistic courage and historical responsibility.  Equally striking was the cosmopolitan character of its Ensemble, bringing together artists from Austria, France, Poland, Canada, Turkey, Morocco, Italy, Germany and…