Do you use proficient with or proficient in?

Well, it helps in general if you have to communicate in English. Writing, speaking, and reading are beneficial on a personal and a professional level. If you are really good, then you might be able...

You could say "nonproficient"; "unproficient" isn't really a "word" (you can say it, but it's unusual).

Yes

There is some confusion here. One talks about a person being proficient in a language. One does not use proficient of things or languages. The is or was a well known certificate in English nicknamed...

deficient, efficient, omniscient, sufficient, beneficient, coefficient, ineffecient, insufficient, self-sufficient