Disappointment at the opera

I don't consider myself a huge opera buff, but I've loved Mozart's Die Zauberflöte (The Magic Flute) since I was a teenager. When I found out that Opera San José---a group whose musicians I've worked with from time to time---was performing it this month, I leaped at the opportunity to see it. I'm a big proponent of enjoying art (vocal works, poetry, movies) in its original language whenever possible, and the website promised "Sung in German, with English supertitles."

I was shocked, therefore, when Tamino uttered his desperate plea, "Ach, schützet mich!" and fell over in a faint, only to emerge from this state speaking English. HUH? Apparently, only the songs were to be presented in German; all dialogue had been translated and was spoken in English. The effect to me was jarring, and I felt a mental grinding, as of changing gears in a car without a fully depressed clutch, every time there was a transition between music and dialogue. It didn't ruin my experience, but my enthusiasm for the performance was undoubtedly dampened.

During the intermission, I asked a staff member why this was done and she told me that it was pretty standard. "If you see this opera in France, they'll do the dialogue in French." I've since confirmed with my friends across the Atlantic that this is indeed common. But my question is, why? The Wikipedia entry for "Libretto" provides a clue:

Just as with literature and song, the libretto has its share of problems and challenges with translation. In the past (and even today), foreign musical stage works with spoken dialogue, especially comedies, were sometimes performed with the sung portions in the original language and the spoken dialogue in the vernacular. However, this reinforces the idea that the words to the songs do not matter, a common misconception in those who do not really understand musicals or operettas.

(Yes, there's some editorializing on the part of the author here, with which I happen to agree.) I understand that opera arias are fiendishly difficult to translate well; this is a subject I've expounded on before. And in the days before projector technology enabled the common practice of supertitles, perhaps opera companies feared that audiences wouldn't want to sit through three hours of an incomprehensible story, so they made the choice to translate the dialogue (a much easier task, since there are far fewer constraints) and leave the music intact, thus giving listeners some clue of the plot. But nowadays, why not just supertitle the entire thing? We don't watch foreign movies with some sections dubbed and some sections subtitled, which is really the equivalent of what happened here.

I'm willing to consider the possibility that my disappointment has partially to do with my love for and competency in the German language. It's not particularly tiring to me to listen to three hours of German, and far worth any increased effort to experience the opera in the beauty of the original language. But would I feel similarly for a Russian or French opera? Maybe I'll appreciate the English next year, when they perform Carmen.