My wife is good....... French.
at is the answer
this at answer is correct because of good in .......... is using with a location; for example good in field/bed. But generally we use good at........within a quality or a subject
This 'at' answer is correct because
Srujan from Hyderabad, India is saying with is correct answer
shafi thayyil from Ernakulam, India is saying at is correct answer