They are from Spain, isn't they? →

They are from Spain, aren't they? → They are from Spain, aren't they?

В вопросительном предложении местоимение 'they' во множественном числе, поэтому вспомогательный глагол 'is' должен быть заменен на 'are'.

Ответ: They are from Spain, aren't they?