Albert Barnes Commentary 2 Kings 18:26

"Then said Eliakim the son of Hilkiah, and Shebnah, and Joah, unto Rabshakeh, Speak, I pray thee, to thy servants in the Syrian language; for we understand it: and speak not with us in the Jews` language, in the ears of the people that are on the wall." — 2 Kings 18:26 (ASV)

The Syrian language - that is, Aramaic; probably the dialect of Damascus, a Semitic language closely related to their own, but sufficiently different to be unintelligible to ordinary Jews.

The people that are on the wall - The conference must have been held immediately outside the wall for the words of the speakers to have been audible.