Yes she is Dutch!
Her name is quite hard to pronounce for English/American people because they don't have the Dutch 'ij' or 'r' sounds in their language, but I'll try my best to explain it: 'Roosmarijn' is pronounced like 'rose-mah-ryan' and 'de kok' is pronounced like 'de (like 'the' but with a d) cock'. Hope this helps
Also: Roosmarijn might seem like a strange name, but it is actually the Dutch form of 'Rosemary'. Also, 'de Kok' means 'the cook' (in Dutch you can have an article in your last name). So an English 'translation' of her name would be Rosemary Cook.
EDIT: I changed the emphasis on the first name; It's 'rose-mah-ryan', not 'rose-mah-ryan'