For he spake unto them by an interpreter - Either there was a very great difference between the two languages as then spoken, or Joseph, to prevent all suspicion, might affect to be ignorant of both. We have many evidences in this book that the Egyptians, Hebrews, Canaanites, and Syrians, could understand each other in a general way, though there are also proofs that there was a considerable difference between their dialects.

And they knew not that Joseph understood them,.... For what is above related they spoke in his presence and hearing; but speaking to one another in the Hebrew language, and he being an Egyptian, as they took him to be, they did not imagine that he could understand them, and therefore were not at all upon their guard in what they said: and what confirmed them in this was:
for he spake unto them by an interpreter; which he the rather chose to do, that they might have no suspicion of him; and which shows, that though there was a likeness between the Hebrew language and the Egyptian in many things, yet in some they differed, and the difference was such that there was need of an interpreter, where the parties did not understand both languages: this interpreter between Joseph and his brethren, according to the Targums of Jonathan and Jerusalem, was Manasseh, the eldest son of Joseph, and so Jarchi; which is very improbable, he being but a child at this time, if not an infant; see Genesis 41:50.