What is the correct usage?

Question

"I am good at English." Or "I am good in English."

Fluent sounds better. But again...

"I am fluent at English." Or "I am fluent in English."

Answer 1

When referring to a school subject we do say at

I am good at Geography

so you could say

I am good at English

If you are describing your capabilities in the language then fluent is probably better and then use in.

I am fluent in English, Spanish and German

We may well phrase it differently, when emphasising some particular aspect of language skills

I speak English fluently