This question at least used to appear regularly on the Thorn Tree bulletin board of the Lonely Planet Web site. I think some said that they found they could use Gol's Argentine Web site and get around the Brazil site's restrictions. You could at least contact American Airlines, since they're a code share partner of Gol. Otherwise, some said they either phoned Gol or used a travel agent. The other major airline in Brazil, TAM, is a bit easier to use since they are a full member of the Star Alliance, plus has international Web sites, offices with toll free numbers in the U.S. (e.g., Miami) and some U.S. sites, such as Expedia, sell tickets on them, charging you U.S. Dollars. But if Gol has the better fare and/or better schedule, I can understand using them. The airline itself is just fine. We're in Brazil, and I've sometimes found Gol cheaper, and sometimes TAM. Both sometimes announce sales on specified routes, usually announcing them on weekends.
It is in English. I just tried a dummy booking and got as far as clicking purchase after entering a VISA credit card number. It appeared as though it was to be accepted. However, since I did not need the ticket, I closed the page.
Another option you might consider if you are looking for bargain flights in Brazil would be to purchase a Brazil Air Pass. If that is acceptable, you must purchase it in the US before you arrive in Brazil. These passes can be purchased from AA, UA, DL and other carriers.