Ergo, they were actually speaking German, as recorded in the movie.
Yes, it turns out that the first few minutes of 'Before Sunrise' are actually supposed to be in German. In fact, the entire film takes place in Austria. Way to read the back of the DVD before watching.
What I really loved about this movie was how, at certain parts, I would laugh because something was so European, or so German (well, Austrian, really). I felt like it was mirroring my experience of moving through Europe in this kind of experimental, self-discovering way--minus the whole love story thing. I know that if I watch it again when I'm back in the U.S., it will make me feel nostalgic and misty for this time in my life.
Plus, I got to feel superior to Ethan Hawke's character, because I could understand what the Austrians were saying and he had to look like a clueless American because he couldn't.