The conversation could have been in Hebrew, but as far as Joseph was concerned that would have given the game away, and in any case he was not keen for the court and people to be reminded of his foreign origins.
The brothers spoke in Hebrew, he spoke in Egyptian, and an interpreter translated. The interpreter’s name is not given in the Torah.
That is the nature of an interpreter: he is not important in himself, only as a conduit.
This message is conveyed by the Lubavitcher Rebbe in a comment on the Septuagint, the Greek translation of the Torah made for King Ptolemy. Tractate Sof’rim (chap. 1) says that the day when the translation was made “was as ominous for Israel as when the Golden Calf was made”. Sof’rim explains that translations are always problematical.
The Rebbe notes that the sages were not equating the Septuagint with the Calf but comparing the two occasions.
The problem with the Calf was that the people were seeking a substitute for Moses. They thought their leader had vanished; they wanted a replacement.
What was wrong with translating the Torah into Greek? Not the act of translating, since it is said that Moses taught the Torah in 70 languages. The sin was in putting the translators on a pedestal, acclaiming them for a great achievement.
Says the Rebbe, “When God chooses an intermediary, the intermediary is ‘invisible’, without independent existence. His only function is to transmit God’s word” (Likkutei Sichot, vol. 24).
In a sense that applies to the rabbinate too. The true rabbi is “invisible”, not important in himself but precious as a conduit.