There are actually various sizing systems here within Argentina and each brand has a much different fit than another. However, last year a law was passed in effort to standardize womens' clothing sizes (in response to the fact that a lot of designers were making only super-small sizes, thus making normal-sized girls think they were fat. This was around the time when there was all the media coverage surrounding the deaths of several South American models).
So...don't even bother trying to convert, really. You just have to pick out what looks just about your size, try it on, and try a next size up or down if it doesn't fit. I've been living here for three years now and I still don't know what my size is (and don't even get me started on trying to find size 10 shoes here!).
There are three systems that I can think of offhand (one including a 0-5 scale. 0 being XS and 5 being XXL). And for bras - I know is that a 36C bra size is a 95 or 100, depending on the brand (no cup sizes here so again, you've gotta try a bunch on).