Yes, the English words of Let All Mortal Flesh are a metrical English translation of the cherubic hymn from St. James liturgy (centuries old Greek, some bits from the Bible) by Moultrie (1864) NOT to the tune PICARDY nor any tune in particular as far as I can find. The tune is a French folk song and those French words (8-9 verses) have nothing to do with Let All Mortal Flesh (4 verses). The folk tune and Moultrie's English poem were married by Vaughan Williams in 1906 for the English Hymnal. There is no "original language" of LAMFKS to sing. The original Greek Cherubic Hymn would not fit the tune. If you'd like to sing (outside of worship) the French folk song, the texts can be found online if you search Jesus-Christ s'habille en pauvre. The only singable English translation I have found of the French folk song are just three of the verses translated by Walter Ehret for the International Book of Christmas Carols. I hope that helps clarify its complicated history.