@Robert

Note:: I think theoretically it suppose to work using the Loco translation plugin. I always get a file size error so don’t even mess with it anymore.

The file you’re looking for is a CORE file and will be overwritten each time the theme is updated. Be careful if you’re unfamiliar:
/DA9/framework/parts/add-form-media.php and this file is called from /DA9/framework/parts/add-form-main.php

Remember to always make a back-up of the files first!