It is strange, considering that they all knew Hebrew, and since Egypt and Canaan were so near to one another how could the brothers not have understood some Egyptian?
One view is that Joseph did not yet want them to know who he really was. It would have given the game away if he spoke Hebrew to them, and in any case their Egyptian may not have been adequate enough to address a ruler.
More probably it was a matter of etiquette. A ruler had to maintain his distance; a foreigner had to show deference. It would not have been correct for the two sides to speak directly to each other.
We learn from this episode that situations dictate the mode of behaviour.
A gabbai in my first congregation realised that he had to establish this ground rule. Many of the small group who came to my Shabbat afternoon shi’ur were personal friends and we were on first name terms.
But one week when Mark asked a question and addressed me as “Raymond”, the gabbai politely told him that at home I was Raymond but in the synagogue I was the rabbi. It is not that I ever believed on putting on airs, but the halachah teaches us that addressing the rabbi correctly one is showing respect to the Torah which the rabbi stands for.
L’havdil, it is something like the lesson I learned when I was a cadet officer at the age of 16 and wore an officer’s cap. A real soldier saluted me in the street and I thought he might have been mocking me, until I realised it was not me he was saluting but the rank and what it stood for.