The standard Korean GPS navigation systems in cars that you get at Herz or Avis are bi-lingual. They also feature a full text search, which was enough for us to find our way around Jeju last year.

The only problem we faced is that e.g. Hallim Park has several entrances and we were unsure which one to chose of the text search results. But, overall it works quite well.

Furthermore, there are plenty of tourist maps and brochures that feature GPS directions (such as phone numbers or numbers nearby) that help.