I think they sound "mannered" (I know what you mean) because we're not always familiar with the mannered tradition in which they sing and perform. It's perhaps a bit like looking through a window at it, rather than being inside the room with it.

I also dislike opera being translated, from the original language for which the music was written, into another language. The rhythms and cadences of the language would have gone more or less hand in hand with the the musical phrasing. I have an old 78rpm recording of Webster Booth (of Anne Ziegler and Webster Booth fame) singing the "Flower Song" from Carmen, with "Your Tiny Hand Is Frozen" ("Che Gelida Manina") from La Boheme on the B side. He had a great voice - but they just don't have the same feeling as when sung in the original languages.