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.
Liberty in matters Scripture neither commands nor forbids. Closely tied to adiaphora, and the reason Lutheran congregations can differ in practice without dividing over it.
In Doctrine — What Lutherans teach, and where it differs from neighbouring traditions.