To which I say, what a load of horse poop.
Set aside that "purity" and "what the composer heard" are imaginary and belief in these goals is a road to fundamentalism. Set aside that until the middle of this century, it was common practice to hear opera performed in the vernacular, so one can hear recordings of Boris Gudonov in German or Siegfried in French. Hell, set aside the existence of Otello and Falstaff and Romeo et Juliette and Faust. Rigidity about original language performance puts a fence around opera, and says "you don't get to enjoy this until you fully understand Italian (or German or French or Czech or...)"
Supertitles are the most common way to try to satisfy the purity police and provide some degree of comprehensibility, but I don't feel like they are adequate. If composer's intent is important, then I'm pretty sure that the composer did not intend for us to be reading during their works. Reading supertitles, the voices on stage are forced to compete with the much less mellifluous voice in your head. If supertitles are the best solution to the problem of language in opera, then the best way for me to enjoy the poem "Eugene Onegin" is to listen to somebody reciting it in Russian while I read a linear translation.
Translation is necessarily inexact; translation changes the sound of the piece; translation interpolates one more artist between the artist and the audience. All true, and all trivial compared to experiencing an opera as it should be experienced, through sound and spectacle. Opera isn't about words, but music (this is sometimes used as the dividing line between opera and musicals). It's true that the sound of English is very different from Czech, but if you're not Czech, then the Czech language sounds like noise (and if you do speak Czech, then listening to an Australian singing Makropolous in the original language probably also sounds like noise). And hopefully, just like the conductor, orchestra, and singers, our translator is an artist. And fortunately, such artists are not rare. I'm pretty sure that the politicians who claim the Bible as their favorite book know very little Hebrew, Aramaic, or Greek. One of my favorite books of all, The Cyberiad by Lem, I know only in translation by Kandel; one of my favorite guilty pleasures, the French Asterix comics, are well enough translated by Bell and Hockridge that some of the puns are actually improved in English.
What spurs this rant is a recent purchase: CD's of Don Carlos and Ariadne on Naxos. These are from Chandos' thoroughly excellent "Opera in English" series, of which I own several and I want to get them all, eventually. The translations are extremely good (there are occasional clunkers, but they're not bad enough to disrupt the flow of things), and having the meaning of the words intelligible as they are sung increases the impact of the music to an astonishing degree. Opera, at its best, gives me the chills or puts a major lump in my throat. Well, I've listened to Don Carlos' Auto-da-fe scene a bunch of times, and Rodrigo and Carlos swearing their friendship, but listening to them and finally understanding the words made them hit me like a ton of bricks. Same for Zerbinetta's big number and the meeting of Ariadne and Bacchus in Ariadne.
It may be, years hence, that I'll have studied enough Italian and German to fully understand these in their original tongues; but until that time, I will probably be listening to these recordings a lot. I have recordings of Falstaff both in English by middle-of-the-road performers and in a powerhouse performance conducted by Toscanini, and I listen to the English version a lot more. Pure or not, I derive a lot more enjoyment and feeling from hearing these operas in a language that speaks to me. Which, when you get to it, is most likely the author's original intent.