I'M SKILLED IN CHINESE
I'M EXPERT IN CHINESE(我是中文专家~专业的刚刚的)
I'M GOOD AT CHINESE
Chinese is my field.
field 为田野 引申义为 领域 , 本行
举个例子 如果 你的妹妹英语学得好 你可以说
It's her field. 这是她的老本行
I"m good at chinese
I do well in chinese或者I am good at chinese.
I am good at Chinese. 或I do well in Chinese.