Or maybe not, because his name was actually Léon, and he was hesitant to write a Christmas song due to the commercialisation of Christmas. Noël wrote the words, influenced by the then-current Cuban Missile Crisis and Gloria the music.
Gloria came into English straight from Latin, and also via Old French glorie to become Middle English glory. I couldn’t figure out what the origin of noël (or noel) might be, and would not have guessed that it comes from Latin diēs nātālis day of birth (compare nativity). French did drastic things to Latin (note also that glorie became gloire), but that one is a stretch. Noël is a relatively late arrival into English, dating from 1805-1815. The First Nowell was first published in 1823.
You say either and I say either, You say neither and I say neither
Most versions of the lyrics online don’t help by not indicating the pronunciation (apparently, neither did the sheet music; Wikipedia reports that Ira Gershwin told the story of one singer who sang the song with the same pronunciations throughout), so I will write:
You say ee-ther and I say eye-ther, You say nee-ther and I say nye-ther
This also doesn’t help by not indicating who is singing, therefore making it clear who says this and who says that. In the movie, he sings this part, so I will write: