Hi,
Do you say 'good at English' or 'good in English'? Which is which?
Thanks.

Neither is especially good writing, but I would say "in" when talking about a specific class, and "at" when talking about overall skills.

Another way might be "good with English"

I would not use either phrase as they seem to be poor grammar. Use: His/her command of the English language is good (assuming you are referring to the language, not the country England or its people). He speaks good English.