At this address is a nice article about opera in English. My recent visit to the Washington National Opera featured an act translated into English which worked well. Placido Domingo performed his aria untranslated, and this serves to point out the main reason for not doing it: the people you are hiring to do the opera, assuming you are talking about the top rung of singers, know the opera in its original language and aren't about to learn it in yours. So do you do without them?
The second problem is that singers are often not understandable in any language so the advantages of translating the text are often lost. It is still recommended to show the supertitles.
I would think the advantages of translation would apply most to comedy so your eyes would stay on the performers and comic business would not be lost. Just a thought.
What do you think?