Yes, both are OK. This discussion appears here probably a dozen times. 'At' is better than 'in' only when speaking of a quite restricted ability (so that it doesn't have 'size' or 'dimension'): 'he's very good at long division.'
Mr. Tom: If you go to "Search the site" and type in "good at/in," you will find many interesting ideas about this subject. You should probably say, "I am good AT English." There is no "rule." Most native speakers have decided to use "at." If you use "in," you are not "wrong." But some native speakers would feel that you are not speaking idiomatic English. ("idiomatic" = natural sounding to a n