I know that several American-born persons have become royals (e.g. Grace Kelly, Lisa Halaby) or heads-of-state (e.g. Eamonn de Valera) in foreign countries. But the said royals have been consorts, not sovereigns; and the said sovereigns were not royal.

Would Pope Leo XIV, then, qualify as the first American to become a foreign royal sovereign -- and if you will, a *king*? After all, the papacy is a monarchy: it's just that the pope is obviously an elected monarch. Indeed, the Chinese word for pope is "Church Emperor".