In my opinion, since these two adult individuals are so well-known, the Spanish names will prevail. (It has been eons since I last saw His Majesty's name rendered as "John Charles" and then I probably only noticed it as a curiousity in context. This MAY occur possibly in exceptionally formal documents rendered into English for an English-speaking country, such as ambassadorial credentials. And even if the document/s required the Royal signature, would you presume that His Majesty would sign as "John Charles I" ?) As for His Royal Highness, how can we be certain that he would choose "Felipe" as his regnal name? AP