As the son of a proud Welsh woman, as a child I was told the origin story of the title "Prince of Wales" was that after Edward I beat Llywelyn ap Gruffudd at the Battle of Orewin Bridge, as part of the peace settlement Edward agreed that the new Prince of Wales would not be able to speak English. The assumption was that this meant he would be a Welsh speaker, and therefore sympathetic to the needs of the Welsh people. Edward I choose to appoint his infant child, abiding by the letter but not the spirit of the agreement.
Looking at wikipedia and a previous quite comprehensive answer there is no mention of this agreement. Also it appears Edward II (the first of the English royal family to hold the title) was not born until 2 years after the battle and not invested until his 16th year. Is there any evidence to support the interpretation I learnt, or is it a nationalistic myth?