Flo King, a rabid Anglophile (Her father was English born, her mother was a Southern Lady) said the title goes back to Edward I, Longshanks, who conquered Wales. His Welch subjects demanded a Welch-born prince, who spoke no English. When his son, Edward (later Edward II) born at Caernarfon Castle, he made him the Prince of Wales. He was Welch born, and spoke no English.
Must be why nobody understood him. In Braveheart, he’s portrayed as a limp-wristed poufta, and in Outlaw King, he’s this virile warrior.