Which preposition is correct in the phrase "proficient in/at/with English"?
With is an instrumental usage, as if English is being considered as a tool -- proficient with knives, proficient with horses, proficient with languages, especially English. Perfectly cromulent, but not often encountered, I suspect.
At is a punctual locative, locating some spot in a larger area or metaphoric space (e.g, in May, on Tuesday, at 2:34 pm) -- proficient at getting his deer every year, proficient at locating the fault in my argument, proficient at language and language games.
In is the general case with proficient (or skilled), though in can't take just any clause or phrase as its object; it has to at least be some activity that is learnable, repeatable, and worth repeating. These examples are terrible, for instance -- *proficient in going down to Joe's and bringing me a ham on rye, right now, *proficient in being late three times out of four, etc.
I'm sure there are many other constraints governing the object of in with proficient, and good luck in discovering all of them.
I generally agree with John Lawler, but let me add this spin:
"proficient in" is generally used when discussing a subject area: "proficient in science", "proficient in auto mechanics", etc.
"proficient with" is used when discussing a tool of some sort: "proficient with a hammer", "proficient with the violin".
"proficient at" is used with a specific activity: "proficient at swimming", "proficient at building houses".