Dutch Queens & Kings

Apart from French kings, foreign royalty is always translated in English. So William of Orange, Frederick the Great, Philip of Spain, Catherine of Braganza etc. etc. To expect "Willem" in this case to the exclusion of "William" is simply wrong.

I enjoyed the quiz even though I only got two - thank you!

It's what type-ins are for. The person typing William obviously knows the answer, even if it's not perfectly translated. Just like you could allow Wilem with one L as a typo. You are right that it's not the name. You can also set the quiz to be Dutch language, which should not allow English type-ins for example.