"I had a fortnight's holiday in Spain."
Translation:Ges i bythefnos o wyliau yn Sbaen.
Yes, depending on the context. pythefnos is used in the same way as the British English 'fortnight' for two consecutive weeks.
(And it is dwy wythos - wythnos is a feminine noun, so dwy, tair, pedair are used with it, rather than dau, tri, pedwar.)