X가 언어를 곰부 해요. Она учит русский язык . There is a feeling of 받다 (receive something) and that is the reason why Accusative case is used - because it answers the accusing questions: accuse who or what? )
Она/он учит Русскому языку - you can translate as X가 언어를 곰부 하게 해요. (feeling of giving something TO someone or forcing someone to do some that is why the case is called DATIVE (from word "дать - give" кому, чему? to whom? to what? ). I think this is why Russians students who just begin learning English translate this sentence in English as "She is teaching to Russian language. " because in Russian you simply don't" teach something", instead you "teach to something".