like an eagle: This comparison shows how quickly the enemy would attack. Just as a bird of prey swoops down suddenly, the invading army would come without warning.
whose language you will not understand: The enemy would speak a foreign language that Israel did not know. The Assyrians and Babylonians spoke dialects of Akkadian.
Akkadian was related to Hebrew, but it was very different in grammar, syntax, and vocabulary (see Isaiah 36:11–13).