“I will punish.” The sentence changed abruptly from 3rd person to 1st person. This kind of change happens quite often in the Old Testament and a number of times in Zephaniah. This is the kind of thing that supports that the book was inspired by a divine Author, God. A professional scribe, and even a fairly well-educated prophet, would not normally write in that seemingly broken fashion.
“the officials.” There are three categories of people mentioned: the officials, the king’s sons, and those people who were wearing foreign clothing. Why not the king? The answer is that Josiah was a godly king.
“the king’s sons.” This is a very important detail because the godly king Josiah was the only king of Judah or Israel to have three of his sons reign as king after him, and all three were evil in the sight of Yahweh. The three were Jehoahaz (2 Kings 23:30); Eliakim also called Jehoiakim (2 Kings 23:34), and the last king of Judah, Mattaniah who was called Zedekiah (2 Kings 24:17).
“clothed with foreign clothing.” Although some commentators think this refers to Judeans who adopted the ways and clothing of foreign lands, especially during Zephaniah’s time in the country of Babylon, that is not likely because God never condemned any specific kind of clothing unless it had pictures or pagan designs woven into the fabric. Besides, the clothing of the common people of the ancient Middle East was mostly all quite similar. One great exception to that was the priests of the various gods and goddesses. The pagan priests, just like the priests of Yahweh, had distinctive garments that set them apart from the rest of the people. For example, at the time of Jehu, the worshippers of Baal had distinctive garments that were specifically used in the worship of Baal (2 Kings 10:22). So there is good evidence that the people who were wearing foreign clothing were the Judean priests. Zephaniah 3:4 says the priest profaned the temple of God, and it would make sense that one of the ways they had done that was by dressing like the priests of pagan gods. There is no reason to assume that the atrocities committed by the priests of Judah that are revealed in Ezekiel were not already being committed in Jerusalem a generation earlier, even if they were not as open or widespread (cp. Ezek. 8:5, 10-17). Josiah’s reform did away with many of the atrocities but did not remove them from the hearts of the people, so they were hidden and then came right back when Josiah was not around.