I tend to give them a duchy under my de jure realm, that tends to bump up their approval of me enough to vassalise them, maybe give them some money. But that just makes them your vassals, you dont get to be a grand master then.
Edit: I am playing an Alanian and I am able to vassalise both the Knights Templars and Knights Hospitalier without having their culture.
Edit: I am playing an Alanian and I am able to vassalise both the Knights Templars and Knights Hospitalier without having their culture.