I think I've heard people use 초급 (beginner), 중급 (intermediate), 고급 (advanced)
급 (as far as I know :p) is usually associate with "class," "rank," or "level"
초 as a syllable/word block can be associated with "the beginning"
중 is often used in conjunction with other words to mean "middle" 중학교 = middle school, or like to construction "in the middle of verbing" through Verb stem -는 중이에요 "지금 가는 중이에요!!" = I'm coming right now!! (actually walking there, taking the bus..... etc)
고 can often just mean height, so together 고급 = something like high class or high level.
So, I think you could say something like this:
한국어로 저는 초급이에요 "In (respect to) Korean, I'm a beginner"
한국어로 저는 중급이에요 " ....... I'm intermediate"
한국어로 저는 고급이에요 ".......... I'm advanced"
Hope it helps! Native speakers could correct me probably :p