If you want it in third person, then I advise the second paragraph be in this way to maintain consistence. Conversely, if you want it in the first person, then change the first part of the text.
"(I am) Brazilian, 21, Christian. Currently studying Economics in Rio de Janeiro State University. Mainly interested in American English and culture. But also open to talk to others English speakers.
If you want to improve your Portuguese I am in to help you in any way I can. And I will certainly improve my English in the process!"
The second paragraph wording was changed a bit, but I know that the essence is there, so it´s up to you wether to change it or not. Thus, my version may constitute, somehow my personal style. But trying to keep it more natural, at least I tried. He hee.
"Mainly interested in American English and the culture from its country.
In my view, you´re interested in American English and American culture, so not necessary here to express that you are interested in the culture from "its" country, which leads the reader to think that "its" refer to the "American English" (not the country as you maybe had thought).
So, rephrasing the way I did, eliminates some ambiguity. That of the reader get himself thinking "he meant -it- the American culture or the country?".
Anyway, other opinions are welcome here, I could be missing something and two heads are better than one, but that is my opinion for the time being.