Forum:The Pit - Finn learns korean?

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I thought since in the upcoming episode The Pit, Lady Rainicorn will appear, I thought that Finn didn't know korean. How could he communicate with Lady? I thought maybe it's another one of those big events such as ones like

Finn and Jake finding out about Ice King's past in "Simon and Marcy"

Flame Princess destroying the Ice Kingdom in "Frost & Fire"

PB and Marcy finally getting along in "Sky Witch"