Looking on the internet deeply has found these results:

I'm good at English is the most popular phrase on the web.

I'm good in English 1122,000 results on the web	I'm good at English 1124,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
someone that is good in English, that know a lot about kids. I'm teaching you proper, good English here, Bashira. I'm responsible for all of the design and production process of our Etsy shop, trendyknitting, and she is responsible of all the communication and listing process on Etsy, since I'm not so good in English. It's not very good in English. It's just the way she said it. The second of all is because I want to learn to speak English or other language better! Im rather good in English though, but my accent is not that good. I've been doing so good in English classes, I'm good in English but math is new to me. You're only good in English, but in other subjects...	Yes. I'm really good at English. I was always good at English. Look, you were good at English. I'm amazed at how good your English is. All the facts I found was in English, but it did not so much because I'm so good at English At Sacred Heart I was good at English. You're supposed to be pretty good at English, even if you are younger than me. I'm amazed at how good your English is. It's perfect.

I'm good in English

1122,000 results on the web

I'm good at English

1124,000 results on the web

More popular!

someone that is good in English, that know a lot about kids.
I'm teaching you proper, good English here, Bashira.
I'm responsible for all of the design and production process of our Etsy shop, trendyknitting, and she is responsible of all the communication and listing process on Etsy, since I'm not so good in English.
It's not very good in English. It's just the way she said it.
The second of all is because I want to learn to speak English or other language better! Im rather good in English though, but my accent is not that good.
I've been doing so good in English classes,
I'm good in English but math is new to me.
You're only good in English, but in other subjects...

Yes. I'm really good at English.
I was always good at English.
Look, you were good at English.
I'm amazed at how good your English is.
All the facts I found was in English, but it did not so much because I'm so good at English
At Sacred Heart I was good at English.
You're supposed to be pretty good at English, even if you are younger than me.
I'm amazed at how good your English is. It's perfect.