The arguments have been made elsewhere many times so, rather than go over old ground, I wanted to add something to consider:
Maybe the reason the argument exists at all is due to the fact that we don't really treat opera as theatre.
Plays written in languages other than English are regularly performed in London (Chekhov anyone?), yet when did you last see The Cherry Orchard performed in Russian with English surtitles? Despite the fact that many of the biggest technological advances in the theatre were brought about to accommodate opera staging, opera is still treated as part of the history of music rather that of the theatre. If you went into HMV looking for a DVD of La Traviata would you look in the Classical Music section or seek out the Shakespeare DVDs and see if it was filed with them? I thought so.
Despite expecting a higher standard of acting (and movement) on modern opera stages, what is expected above all is beautiful singing. I've been going to opera for the past 10 years and, in that time, have seen three different casts of Turandot - none of the actresses in the lead role have been dramatically believable, yet we can get past that if the singing is good. Given that the average dramatic voice doesn't settle til well into a singer's 30s and that most soprano heroines aren't anywhere near that age, is it really so surprising that we can suspend our disbelief for opera in a way we never could for other forms of theatre?
Most opera fans I know are happier to listen to a recording than watch a DVD (and an opera is generally no more expensive on DVD than CD theses days). Music - that's what it's all about and that's why opera will never be treated in the same way as theatre or for the same reasons.
Just a thought - I waver all over the place with this topic. I'd love to understand Wagner, Puccini, Mozart and Janacek in their original languages but I can't. I do own a few recordings of opera in translation and some are better than others - the quality of the translated libretto makes a huge difference. I still generally prefer opera in the original language though and see most of my opera at Covent Garden rather than ENO (although ENO do some of their best work with Britten operas, which were written in English).
Everybody's got their own opinion on this - my favourite comment in favour of opera in translation comes from John McWhorter, writing in The New York Sun in July 2008:
"Vowels are nice, but they aren't everything, and for my money an evening of vowels and reading is not much of anything - especially for a hundred dollars and change".
Add a comment and let me know what your opinion is.