On: every psalm and song in the daily prayers uses today’s English. Off: all of them use the older “thee and thou”. Until you choose, the Songs of Zechariah, Mary and Simeon use today’s English, as most churches sing them. The Lord’s Prayer and the prayers for the dying show the form most people learned.
Luther's own confession, written when he expected to die and wanted his position on record without Melanchthon's diplomacy. Blunter than the Augsburg Confession, and the clearest thing he wrote about what he would not concede.