Al Remsen's A Christmas Celebration in Song and Story,1999. Explains the "Portuguese Hymn" this way...

Since England had rejected Papal authority, there were few locations where Catholics could celebrate the mass. One of the few was the chapel at the Potuguese embassy in London. It is thought to have been brought to the embassy by English pilgrims returning from Douay in Northern France. At the Roman Catholic center for Englishmen in Douay was a resident copyist and music teacher named John Wade, who wrote the Latin verses in 1731. It's unclear if he wrote them or transcriberd another source, but most scholars credit him.

So an Englishman, working in France, writing in Latin created the hymn, which returnt home to England via the Portugese embassy....

Tinker