Probably useful if you are a Katusa. Air Force may have their own standards in translating. Navy and Marines obviously have their own thing going on too. 

* Specialist and CPL are of same pay-grade and rank hierarchy, but Corporals may lead squads while Specialists generally don't. They are generally miserable in their daily life because they don't think getting Specialist pay for Sergeant-level work is worth it. 

** You will refer to Sergeants, Staff Sergeants, and Sergeants First Class (that aren't acting First Sergeants) as simply "Sergeants" verbally. Drill Sergeants are referred to as Drill Sergeants. 

Executive Officer / XO = 부관. Use the unit the XO is in charge of in case of ambiguity (Battalion XO, Alpha Company XO, Brigade XO, etc.)

Commander = 사령관

Chief = Short for Chief Warrant Officer (준위). You see Chief 1 or Chief 2 at Battalion level, Chief 3 at Brigade, Chief 4 and 5s at beyond. You will almost never see a Chief 5 unless you work in USFK HQ or something, although Chiefs can be posted down a level or two if they want to. You salute Chiefs like you salute Commissioned Officers.