Comma placement dilemma

Hi.

I am confused about the placement of a comma in  situations like these.

Many have an excuse, "I am not good at English."

To the question, "Is your English getting better?" he answered, "Yes, it is."

However, the excuse, "I am not good at English" is not needed under these circumstances.

Sorry, another question.

Should it be "I am not good at or in English"?

Thank you, Mr. M.

Can you help me to understand it a little better?

Based on my understanding this are possible, although it is quoted from what someone else has said. I think when someone sais something is a quoted phrase, it is not limited to instances of rewriting what someone has said.

The expression "I am not good at English" is just as correct as "I am not good in English." -- Is the apposition in place here. I think there is not since there  isn't a comma. Then why a  comma is needed here below? What factors are involved in making an apposition possible in a typical situation?

However, the excuse, "I am not good at English", is not ...