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; Psalm 2:4; Psalm 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.

Isaiah 28:11 Parallel Commentaries

The Husbandman and his Operations
'Give ye ear, and hear my voice; hearken, and hear my speech. 24. Doth the plowman plow all day to sow? doth he open and break the clods of his ground! 25. When lie hath made plain the face thereof, doth he not cast abroad the fitches, and scatter the cummin, and cast in the principal wheat and the appointed barley and the rie in their place? 26. For his God doth instruct him to discretion, and doth teach him. 27. For the fitches are not threshed with a threshing instrument, neither is a cart wheel …
Alexander Maclaren—Expositions of Holy Scripture

How to Make Use of Christ for Steadfastness, in a Time when Truth is Oppressed and Borne Down.
When enemies are prevailing, and the way of truth is evil spoken of, many faint, and many turn aside, and do not plead for truth, nor stand up for the interest of Christ, in their hour and power of darkness: many are overcome with base fear, and either side with the workers of iniquity, or are not valiant for the truth, but being faint-hearted, turn back. Now the thoughts of this may put some who desire to stand fast, and to own him and his cause in a day of trial, to enquire how they shall make …
John Brown (of Wamphray)—Christ The Way, The Truth, and The Life

Of Orders.
Of this sacrament the Church of Christ knows nothing; it was invented by the church of the Pope. It not only has no promise of grace, anywhere declared, but not a word is said about it in the whole of the New Testament. Now it is ridiculous to set up as a sacrament of God that which can nowhere be proved to have been instituted by God. Not that I consider that a rite practised for so many ages is to be condemned; but I would not have human inventions established in sacred things, nor should it be …
Martin Luther—First Principles of the Reformation