The usual expression uses "at".
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Odessa DawnWhen you say the usual expression uses at, does that mean it is not incorrect, so is I am not good in English, please?For me, "good in ..." is not very natural. However, if you search Google Books, for example, you will find examples of this pattern from competent native writers.
jon8 The only expression that I can think of that uses "in" is "I'm good in bed".I would say that "good in bed" is slightly different because "in bed" has a literal meaning. In other words, "bed" is not a subject or endeavour or activity.