In Ancient and Medieval times, did rulers often need translators to communicate with rulers of a foreign language? If so, could translators abuse this? Or were the rulers quite literate in foreign languages?

If there are any experts on Frederick II around I'd love to hear about him specifically. I've heard that his ability to speak Greek and Arabic was very important to his success, and that the return of Jerusalem during the 6th crusade could largely be attributed to his ability to negotiate in Arabic. Is there any truth to this? Was diplomacy significantly hampered between Frankish and Arabic rulers during the time of the Crusades simply due to bad interpreters?