Our marriage witnesses are two foreign men. Does their nationality affect the validity of the contract?

No; nationality is not a condition for marriage witnesses in Islamic law. What is required is that the witness be Muslim, male, of age, of sound mind, free, upright, and able to hear and understand the words of the contract. So two Muslim men from another country can be valid witnesses if they meet those conditions. Make sure they understand the language used during the offer and acceptance. However, religious offices in Malaysia may have their own administrative requirements, for example valid identity documents to be recorded in the marriage register. Ask the state Islamic religious affairs office where the contract will take place what documents the witnesses need to bring. See conditions for marriage witnesses.