Add-ons can be modified through (Products - Edit - Manage Add-ons - Select Add-ons Group - Add (as in the following image) - Record the name of the add-on in both languages, then Save the add-on) and then record the rest of the add-ons and click Save for the product.

Note that the add-on to the product is currently only the name of the add-on, and when you select it, the dates are displayed below it, and when you follow the previous steps, the add-ons appear directly in the list.