Should I mark it as a language I am proficient in in the Common App?
“Proficiency in a language is about how comfortable you are using another language as a form of communication. In order to be proficient, you must be able to communicate effectively and converse comfortably.” (from Common App website)
Basically, that means being able to carry out an intelligent conversation with another native speaker. Considering it is your first language, yes do it.
I agree, list it. It gives you a slight edge in the diversity factor.
Thank you both 