I knew no Spanish when I arrived (alone, and female and 20), just a couple of words of Italian, so my first day or so was spent saying grazie and prego to bemused locals. I enrolled in a Spanish language school for two weeks. That gave me the basics, plus I met some people I'd later travel with. If I wanted to meet more people/ speak English for a while, I'd just head to a beach on the backpacker trail. Despite having always been painfully shy (only child, bookish), I found it really easy to make friends when traveling (and have done ever since, in fact. I guess it served as a lifelong confidence boost).

That said, I'd not recommend Mexico now - it is heartbreakingly violent at the moment.

I would recommend Argentina and Uruguay! I actually just got back to Europe after a year in Argentina (for work, not backpacking). It's fascinating - the whole sensual-Latin-fiesta thing and, at the same time, a populace that is more self-analytical than Woody Allen and more literary than the NYRB... :) I wasn't doing much traveling, but the provinces I did visit seemed very safe and friendly to foreigners.