I looked in descr_cultures, but I couldn't make sense of it. Then I looked in export_descr_buildings, and I found this for the ports:
port city requires factions { northern_european, moors, turks, egypt, kwarezm, mongols, eastern_european, greek, southern_european, slave, }
Below that it says that the Cumans can recruit ships from them, but can the Cumans build ports? Are they included under the eastern_european umbrella? After reading this thread I want to start a new campaign as the Cumans, but first I want to make sure that their port building isn't bugged, since the last time I played them I couldn't build ports.