The western European languages that evolved directly from Latin, e.g. Portuguese, Spanish, Catalan, Italian , and even French, share mostly the same general grammar structure and a common core vocabulary, although that may not be immediately obvious for non-native speakers. So, if you learn one of them, learning another in the same group becomes much easier.
The Germanic languages, on the other hand, tend to be more dissimilar, but knowing both English and German, it should be relatively easy to learn Dutch, and easier to learn a Scandinavian language.
I hope William's kids have better opportunities to acquire foreign language skills than their father, who is pretty poor in that respect.