Is it permissible to take a 15% commission from the value of every work completed by a translator brought to the site, on the basis that this serves as an incentive to search for translators?

It is not permissible to deal with the website in bringing translators based on that agreement due to the uncertainty of the wage in it, which is a percentage of the value of each work completed by the translator who comes from your side. The wage or fee must be known for the validity of the ijarah (leasing/hiring) or ja'alah (reward/stipend) contract, according to the majority of scholars. Therefore, the contract with the website should be rectified so that the ja'l (stipend/fee) is known and specified, for example: that for every translator you bring, you are entitled to a ja'l of a certain amount.