Pardon?

Today a woman sat next to me on the train. After she sat down she asked me a long, meandering question in french about how the trains work in Switzerland (like with seat reservations and so on, not about the mechanics of trains), I got confused and we muddled through in some french and some english and eventually we answered her question. As it turned out that wasn't her only question, though, and since my french was better than her english, we blathered on for quite a while longer in french with a significant amount of success (and who knew I knew so much about trains?).

When we got to Fribourg I got off the train, and nary two steps onto the platform, some woman accosted me and asked if this was the train to Thun. No, I said, it goes to Berne and Zurich, and then I thought "I think I may have just said something in complete gibberish" but the woman looked satisfied with my answer and so off we both went and a few moments later it occurred to me that she had asked me in german and I had replied in german.

Which is hardly earth-shattering, but I was shocked. In my french classes I am forever agonising over whether what I am saying is french or german (or some sort of half-hearted mongrel non-language). Apparently the solution is to not give it any thought at all.