Or HaChaim on Genesis 42:23/אור החיים על בראשית מ״ב:כ״ג

1 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.

2Alternatively, they could not speak freely being afraid that the interpreter would overhear them.