VerygoodEnglishwouldbewrittenas"excellentEnglish.""IamnotverygoodatEnglish"couldbewrittenas"MyEnglishskillsarenotverystrong"or"IamnotproficientinEnglish."
Very good English would be written as "excellent English."
"I am not very good at English" could be written as "My English skills are not very strong" or "I am not proficient in English."