They can communicate with each other language-wise
They most probably won't want to talk to each other, but this is another kind of communication barrier.
I have serbian and croatian friends, once I made an attemt to introduce a croatian girl to another serbian girl, same age, both super nice and communicative, both at the Uni, common interests etc.... it was the fail of the year... it was glacial and they just went to 180° directions just after that.