"King" is widely used by Japanese people^^
yeah but if their calling him lord of the flames ~ i think they'd use the kanji 王 for the lord part.
im not a japanese expert, i just did a couple of japanese courses at university but it seems to me that "キング" part refers to king's name and not "lord" since its in katakana and the name of the person in question is king.
king's original title was 火災のキング (kasai no king ~ which was translated as king the wildfire or king the conflagaration and not lord of the wildfire or lord of the conflagaration).
so similarily his new title that marco said 火炎のキング (kaen no king) should be translated as king of the flames or king of the fire.