Isaiah turned his critics" words back on themselves; what they had said about his words in mockery would overtake them. If God"s people refused to listen to words spoken in simple intelligibility, He would give them unintelligibility as a judgment (cf. Matthew 23:37). Since they refused to learn from a prophet who appealed to them in their own language, He would teach them with plunderers whose language (Akkadian) they would not understand, but whose lances they would take in. They would learn to rest on Yahweh from their foreign foe"s treatment of them if they refused to learn that lesson from Isaiah. [source][source][source]
The Apostle Paul used Isaiah 28:11 to remind the Corinthians that messages in tongues (foreign languages), far from being a sign of spirituality, indicate that the recipients are spiritually immature ( 1 Corinthians 14:20-21). Likewise, Isaiah revealed that when people are so spiritually dull that simple messages do not move them, God will teach them through experience. [source][source][source]