You develop situational languages, and I think one of the big divides that develops is public vs. private. After living in Korea, we stayed with my sister in Austria for a month before coming back to the US. We both had tourist German, but the first language out of our mouths at the store was always Korean. We confused a lot of people. We did a trip to France at the same time, and my husband (low-level French) had the same problem, but I fell naturally back to the kind of French I spoke when I was studying it at university level. Ten years later, French is my default 'foreign language' response in normal circumstances, although rather atrophied. It's fun how brains work.