From at least the early Middle Ages royals have chosen or been required to marry other royals, who have been almost inevitably foreigners.
For example, Spain came to be ruled by Habsburgs, who were German, and then by Bourbons, who were French. Elizabeth of England's rival, and sometime brother-in-law, Philip II of Spain, had only one Spanish grandparent, and, being blond, he took after his Flemish / Belgian grandfather.
The British monarchy is no different. The Queen can trace her descent from the Saxon king Alfred, heroic defender of Wessex against the Danes and also from the 11th-century Scottish king Malcolm Canmore - but there have been rivers of foreign blood since. Royalty are among the most successful of immigrants.
The last monarch to have been as much as half-English by birth and heredity was Queen Anne (1702-14). Her father, James, Duke of York made what was considered a misalliance, marrying a commoner, Anne Hyde, whose father had been a mere lawyer, though as Charles II's chief minister he was created Earl of Clarendon. None of Anne's 17 children lived to adulthood. So when she died in 1714, it was necessary to import, as her nearest Protestant relation, George, Elector of Hanover. The British monarchy therefore became German.