Actually, they can marry someone from another country. They've done it for centuries. It's just that since George and Mary they haven't. (Although one could argue that Mary wasn't a British Princess) and then there's the whole Philip was a Prince of Greece. It was on the guidance of his uncle, Lord Mountenbatten (himself a member of a former foreign ruling house) that Philip renounced his titles and became a British citizen (which he didn't have to do, as he was already one because of who he was a decendant from. Electress Sophia).
As it stands right now, William just has to marry a non-Catholic. There's no other offical rule.
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