The question I ask myself when I open the music and start grumbling is, "Does providing a singable English version of a well-known piece limit or augment that composition?" Earlier just today I found a Brahms motet with both English and German in the score. My knee-jerk reaction is frustration that people don't learn other languages more often. Usually any English you find in an art song (speaking of solo literature now) is not something you'd ever sing, because it is usually poetic and doesn't have the preservation of the original meaning of the text as its primary goal. I could write lyrics that fit the rhythm of the music Brahms wrote, but singing those lyrics--would that really count as having sung the piece Brahms composed?
Struggling with other languages--let's be honest, it's not a cakewalk to learn to sing in another language--builds character, I think. You have to learn how to appreciate another set of phonemes. You don't have to learn the grammar or the syntax or anything much about the actual construction of the piece in order to sing it well, which is another reason I think singing in foreign languages is so cool--you don't have to spend years studying before you can say things!
You also get a chance to experience, in a way, other cultures. There are differences--sometimes subtle ones--in the ways our words shape our vision of the world. Now of course you have to put in the effort and really live with the text in order to reap some of these rewards, but even just exposing our ears to the sounds of other cultures, I think, can really do us good.
But, then, does singing a piece in a language other than the original language actually diminish its value? There are tons of books that have been translated into languages other than the one the author used initially. Does reading those books in their translated forms mean that you haven't REALLY read the book? I'm not sure whether I would say yes or no to that question, so I am not sure I can say yes or no to the same question when asked of singing. You certainly have to spend a lot of time with a language in order to be able to read and understand it, so I can definitely see a lot of value in translating books and texts so that people everywhere can understand what you want to say without having to learn your language first. So what is it about English versions of, say, the Agnus Dei that bug me so much?
Maybe there are so few translations that are both singable and true to the original text that my view of all of them has become tainted. At the same time, maybe I feel like not taking the time to learn how to pronounce the words--or at least to TRY to pronounce them!--feels like laziness.
Do you think I am missing the point of English translations? Would you say that they are more for the sake of making the meaning of the piece accessible to the audience and to the performer and are therefore rather valuable, or would you say that despite the translator's intentions, English translations are less desirable than singing in the original language? (As with all things, I don't think this is a black-and-white answer, but I am curious as to where the majority of the truth falls.)