Jamieson, Fausset and Brown on Isaiah 36:11

11. Syrian--rather, "Aramean": the language spoken north and east of Palestine, and understood by the Assyrians as belonging to the same family of languages as their own: nearly akin to Hebrew also, though not intelligible to the multitude (compare 2Ki 5:5-7). "Aram" means a "high land," and includes parts of Assyria as well as Syria.

Jews' language--The men of Judah since the disruption of Israel, claimed the Hebrew as their own peculiarly, as if they were now the only true representatives of the whole Hebrew twelve tribes.

ears of ... people on ... wall--The interview is within hearing distance of the city. The people crowd on the wall, curious to hear the Assyrian message. The Jewish rulers fear that it will terrify the people and therefore beg Rab-shakeh to speak Aramean.