2 Kings 18:26
Then said Eliakim the son of Hilkiah, and Shebna, and Joah, unto Rab-shakeh, Speak, I pray thee, to thy servants in the Syrian language; for we understand it: and talk not with us in the Jews’ language in the ears of the people that are on the wall.
The verse belongs to the history of the remaining kingdom of Judah after the ten tribes had been swept away. The surrounding chapter records Sennacherib's invasion of Palestine and siege of Jerusalem, an event corroborated by his Assyrian cylinder. That cylinder also mentions a misfortune in his camp, but it does not record the loss of 185,000 soldiers. Eliakim the son of Hilkiah, and Shebna, and Joah asked Rab-shakeh to speak to them in the Syrian language, which they understood, and not in the Jews' language in the hearing of the people on the wall, because the Assyrian officers were speaking directly to the people in their own language in order to influence them.
Rabshakeh's use of the people's language was meant to frighten them. It was meant to undermine their trust in Hezekiah. It was meant to persuade them that Jerusalem's God could not save them.