I want to be as fluent in English as I am in my native language.

[tufguy]

1) I want to be as fluent in English as I am in my native language.

2) I want to be as fluent at English as I am at my native language.

3) I am not good "in" or "at" maths.

We are usually good at something or good at doing something. Is it correct? Could you please tell me how to determine where we have to use "at" and where we need to use "in"?

 

[jutfrank]

I want to be as fluent in English as I am in my native language.

I am not good at maths. [to be good at something]

 

[tufguy]

I want to be as fluent in English as I am in my native language.

I am not good at maths. [to be good at something]

Okay if we are saying "good" then we have to use "at" otherwise "in" can be used.

 

[jutfrank]

Okay if we are saying "good" then we have to use "at" otherwise "in" can be used.

No, that's not a good rule to remember. You can't generalise like this. Just try to remember individual 'chunks', and their uses, such as:

to be good at (doing) something
to be fluent in a language