What is the ruling on working for a company that sends students to study in Germany, where my role is limited to preparing advertisements for university studies in Germany?

There is no objection to sending students to study at Western universities for their formation and to teach them what the Ummah needs, provided it adheres to Sharia guidelines. If that is the case, then there is no objection to working with the entities responsible for this, whether the work is in the field of promotion or otherwise.