[digitalengineer]

Very true! In my part of the Netherlands Dutch is the official language, but the much older and local other language is Frisian. What you see often is students talking Frisian to other Frisians and Dutch to non-Frisians even during the same group-conversation. They don't mean to, but can't help themselves. The "other mind" takes over seamlessly. I've seen Non-Frisians kindly ask if they can just talk Dutch only to hear them speak Frisian after about 20 seconds.