This could be a definition of a professor's nightmare, standing in front of well over a hundred students, freshmen and graduates combined, without a prepared lecture.
It turned out, that I could handle it all right.
I did it by focusing the lecture on the concepts of "language" and "communication" in general, pointing out the potential problem points for cross-cultural communication in particular as we were passing them by. It was also lucky that both lectures were supposed to be inroductory. But still, I'm sort of proud of myself.