Glory to the newborn king!
Like most of the Christmas songs that we are familiar with, this was written in the mid-1800s. And yes, when someone in those days wanted you to listen, they said "hark!".
A herald is someone who proceeds an important person to gather up attention.
Peace on earth and mercy mild
God and sinners reconciled!
The only time the world "reconciled" is used these days is when you are comparing your checkbook entries against your statement from the bank. It just means getting two things to agree with each other. It seems a strange word for a song lyric, even back then, but it does rhyme.
Joyful all ye nations rise
Join the triumph of the skies
These lyrics were written in such a way as to make them seem much older. People in the 1800s didn't say "ye" - nor did they say "ye" in the 17 century. The word was "the". "Ye" was only a written abbreviation, the word was always "the". If you look at 17th century manuscripts you will see a "y" used to replace "th" in "ye", "yet", etc. It was just a quick way of writing a "th". Apparently by the 1800s people had forgotten this. So, anytime you see the word "ye" used, as in "ye old pub", it's a fake.
With angelic host proclaim
Christ is born in Bethlehem!
Still, it's a beautiful song, and a beautiful lyric.
Hark the harold angels sing