I would say that properly speaking, the opposite of "not in English" is null. Pick any sentence with "not" in it, and to form the opposite you simply remove the "not" and leave nothing in its place.
He is not here.
He is here.
I do not like broccoli.
I do like broccoli. (Or simply "I like broccoli," in which the null replaces two words.)
This is rather common. No in Spanish, nye in Russian, ne... pas in French, bu in Mandarin. Remove the negative from the sentence and leave the place blank, and you've got the positive.
Still this rule is common but not universal. In Japanese:
Ni Hon go ga deki masu. I speak Japanese.
Ni Hon go ga deki masen. I do not speak Japanese.
"So" is a word expressing emphasis or denial, not simple negation:
You are not smart.
I am so!
Other words fulfill the same purpose, such as "too" informally and "indeed" formally. But none of them is truly the opposite of "not."
If I said offhandedly, "I have not been to Poland," you would surely respond "I have been to Poland," with no need for a word of emphasis or denial.
Marilyn has the world's highest verified IQ score, but that does not make her a linguist any more than it would make her a chemist.