“订立合同”常见的英文表达是 “enter into a contract” 或 “conclude a contract”。
enter into a contract：强调进入合同关系的过程，是比较正式、常用的说法。例如：The two parties have entered into a contract for the supply of goods.（双方已就货物供应订立了一份合同。）
conclude a contract：侧重于达成、完成合同签订这个动作，也有“订立合同”之意。例如：After several rounds of negotiations, they finally concluded a contract.（经过几轮谈判，他们最终订立了一份合同 。）