וְהֵם֙ לֹ֣א יָֽדְע֔וּ כִּ֥י שֹׁמֵ֖עַ יוֹסֵ֑ף כִּ֥י הַמֵּלִ֖יץ בֵּינֹתָֽם׃
And they knew not that Joseph understood them; for the interpreter was between them.
Rashi on Genesis
והם לא ידעו כי שמע יוסף AND THEY KNEW NOT THAT JOSEPH שמע UNDERSTOOD their language, and they said this in his presence.
Or HaChaim on Genesis
והם לא ידעו…כי המליץ בינותם. They did not realise this….for the interpreter was between them. The brothers who had observed the interpreter all the time, assumed that Joseph did not understand Hebrew.