For - This verse is to be understood as a response to what the complaining and dissatisfied people had said, as expressed in the previous verse. God says that he will teach them, but it should be by another tongue - a foreign language in a distant land. Since they refused to hearken to the messages which he sent to them, and which they regarded as adapted only to children, he would teach them in a manner that should be “much more” humiliating; he would make use of the barbarous language of foreigners to bring them to the true knowledge of God.
With stammering lips - The word which is used here is derived from a verb (לעג lâ‛âg), which means to speak unintelligibly: especially to speak in a foreign language, or to stammer; and then to mock, deride, laugh at, scorn (compare Isaiah 33:19; Proverbs 1:26; Proverbs 17:5; Psalms 2:4; Psalms 59:9; Job 22:19). Here it means in a foreign or barbarous tongue; and the sense is, that the lessons which God wished to teach would be conveyed to them through the language of foreigners - the Chaldeans. They should be removed to a distant land, and there, in hearing a strange speech, in living long among foreigners, they should learn the lesson which they refused to do when addressed by the prophets in their own land.