When I'm in Vienna, I always watch the repertoire in advance, I've been to the operetta twice, both times consciously. Even the entry level of German was enough, there are subtitles in English, the theater is very cozy, the orchestra is good. I highly recommend it.
A very interesting, constantly updated repertoire! The performances are absolutely "piece-by-piece"! The theater is not pompous, it is very popular with opera lovers.