Go to Punta Del Este Uruguay.
It's a fairly educated Spanish speaking country, and I had no problem speaking English there (I don't speak Spanish). This is the beach that all of the rich Argentinians go to. The city speaks English fairly well, and it's the cleanest and most modern country I've been to in South America.
The best part about going here, is it's only an hour long flight from Buenos Aires. So you can spend a couple of days there seeing the Paris of South America.
Another option... If you're not from the USA is Cuba. Fantastic beaches, and although it's not in South America, a good portion of it's residents are ex-South American so the culture is alive and kicking. Add to that the fact that they all learn two languages in school... so they all speak Spanish, and most of them speak English as well (some speak Russian still).
Have fun!
