Where the sale of goods is between two businesses in different Contracting States, then it is international and the CISG rules of law automatically apply unless they have been excluded. A contract clause stating that the contract is subject to the legal systems and courts of a particular country overrides or excludes the CISG rules, since the local rules of that country would apply instead.
So the answer should be Canada's legal system.
Reference: CIPS study guide page 49-52
LO 1, AC 1.2