Hertz licences KT (which is Kumho) at Jeju - that's where we got our car from. It's a short shuttle bus ride from the airport. The nav system was bi-lingual. And, I hope to remember correctly, the GPS rental in Jeju was free and in every car. Staff were very helpful, patiently explained the nav and helped to enter our first destination. The GPS is ok; nothing special and has its shortfalls - it was fine for us. (Trip was in Nov last year)

Avis is AJ (at least at ICN); their nav was the same, so also bi-lingual.

Another option is renting a Mifi at Jeju Airport (at least in Seoul very cheap) and use your cell phone to navigate. As you can pre-set routes and have internet in general, this might actually be convenient in any case.

Overall, navigating your way around Jeju is pretty easy and sights are clearly signposted on the road. Speed cameras are atrocious; cruise control is a nice feature to have in the car.