I obviously don't master the Esperanto language (or "have a good command of Esperanto") since I'm not even sure how to say that in Eo.
My guess is mi mastras Esperanton, which also reflects German ich beherrsche Esperanto and French je maitrise l'espéranto.
But according to PIV, the meaning of mastri is "regi, direkti, kiel mastro". Then, mi mastras Esperanton sounds like I am a kind of Esperanto cifal (like the Volapük cifal). Is this expression nevertheless correct?