There is a phenomena that exists in Catalunya which many Australians probably find quite strange, although it’s far from unique, especially in Europe.
Kids grow up here speaking two languages, they’re similar to be sure, but they are distinct.
Since many, but not all, of the people they will communicate with speak both languages, how to determine which one to use.
Obviously preferences are involved, but if they go in different directions, how to choose one in any given circumstance?
Most catalans I’ve met have a fairly simple rule.
One person, one language.
They speak a given language with a given person, and in many cases they find it very difficult to speak the other with that person.
Siblings especially. Having grown up speaking catalan to one another, appear to be almost incapable of holding a conversation in castilian (spanish).
I considered this fairly normal at first, an acceptable condition, I understand the strangeness of speaking castilian to a fellow english native speaker.
But I also have friends who seem more than capable of changing the language to suit the group.
A group of italians who switch to castilian the moment I show up.
A german friend who carried on a conversation with his sister in english because I was present.
So, maybe it’s not actually a normal condition, maybe it’s artificial.