“Operetta, once the most popular European form of musical theater, has declined in its native lands, while in the United States it never caught on strongly. Today Johann Strauss’s ‘Fledermaus’ and Franz Lehar’s ‘Merry Widow’ are the only two operettas Americans are likely to have encountered, apart from the light operas of Gilbert and Sullivan.” A festival called Mostly Operetta, presented this month in New York by the Austrian Cultural Forum, updates the aging art form and puts it in an intimate space.