בראשית מב:כג פירוש: רש״י, רשב״ם, אור החיים ורד״ק

והם לא ידעו כי שמע יוסף AND THEY KNEW NOT THAT JOSEPH שמע UNDERSTOOD their language, and they said this in his presence.

כי שומע יוסף, he understands, [not only hears. Ed.]

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.