“he will speak.” God will speak to Israel by the Assyrians, who attacked and conquered the land of Israel and carried the people captive back to Assyria. God had been speaking to Israel through His prophets since its inception to return to God. During the reign of Israel’s first king, Jeroboam I (this is the Northern Kingdom of Israel consisting of the ten northern tribes of Israel), Ahijah the prophet foretold that Israel would be uprooted from its land and carried beyond the Euphrates River, an area that, in Isaiah’s time, was ruled by Assyria (1 Kings 14:15). Since the Jews ignored God’s prophets who spoke to them and told them to repent, God then “spoke” to the Jews through an invading army that did not speak Hebrew, which is the meaning of the “stammering lips and another tongue (another language).
God promised that if His people obeyed Him then they would defeat their enemies (cp. Lev. 26:2-8). The fact that Israel was defeated by their enemies was thus a “sign” that they were sinning and living apart from God’s favor. God uses this example of the Assyrians with their foreign language being a “sign” to Israel to good effect in 1 Corinthians 14:21-22. [For more on the “sign” of the Assyrians, see commentary on 1 Cor. 14:22].