As an interview question, "What's your favourite language?" is only interesting if you don't ask it having a "correct answer" in mind.
Depending on when you learn that you have to leave, you can either say "Sorry, I won't be able to stay for the whole duration of the interview" or "Sorry, I'm needed elsewhere, my colleague will handle the rest of the interview". Leaving without saying a thing (even if it's just a nod and a smile) is more disruptive for the candidate than saying nothing.
Depending on when you learn that you have to leave, you can either say "Sorry, I won't be able to stay for the whole duration of the interview" or "Sorry, I'm needed elsewhere, my colleague will handle the rest of the interview". Leaving without saying a thing (even if it's just a nod and a smile) is more disruptive for the candidate than saying nothing.