It's not Brazil or Argentina, it's not a competition between countries, there is no competition and it is people from those countries that buy and sell coins. A Brazilian buying coin doesn't mean Brazil as a country is buying, BMW accepting coins doesn't mean Germany is dealing with Bitcoin.
Probably, this is the leading exchange:
https://coinmarketcap.com/exchanges/negocie-coins/
Those numbers are so weird.., I'll look around for some other exchange but I haven't yet seen one with a 100 000x difference between the top trading pairs in volume
This is negociecoins:
This is mercadolibre:
And another exchange, bitcointrade