Commentary for Genesis 50:6

כאשר השביעך ACCORDING AS HE ADJURED THEE — For except for that oath I would not permit you to do so. He did not dare, however, to say to Joseph, “Break the oath”, lest he might retort, “Then, I, too, may break the oath I made you that I will never reveal that secret about the Sacred Tongue, viz., that I possess a knowledge of it in addition to the seventy languages which you also understand but of which you have no knowledge at all — as it is related in Treatise Sotah 36b.

Not to reveal about the Hebrew Language... Question: If so, how was Yoseif allowed to speak to his brothers in Hebrew? Rashi explained [that Yoseif spoke to them in Hebrew] on, “I speak to you with my own mouth” (45:12). The answer is: Pharaoh stipulated with Yoseif not to speak Hebrew specifically to the Egyptians. When Yoseif spoke to his brothers, no one else was with him in the house, and it did not become known that he spoke to them. (Re’m) The Nachalas Yaakov writes: The real question is that everything Yoseif’s interpreter spoke [to the brothers] was in Hebrew, and many Egyptians were present. Furthermore, Re’m contradicts himself, because he wrote on 45:12 that many Egyptians knew Hebrew [since it was the language of the neighboring land of Canaan]. If so, what use was it for Yoseif to swear not to speak in Hebrew, when many Egyptians were speaking it [even though Pharaoh did not understand Hebrew]? See his elaboration there.