I remember a lesson as to the usages of prepositions that when we are talking about our proficiency in a certain activity with the word 'good', we should use at. The preposition 'in' would be used when we talk about someone's competency in a specific field or role. Nevertheless, I believe that a specific rule in regard to this usage can't be formulated. Correct me if it doesn't make any sense. 

thankful my dear friends to answer my question

"Good at" english (or any other language), this is a collocation

i think in english is true