Spain and South American countries have been trade partners since South American countries gained their independence in the early 1800s. Cultural and emotional links between “La Madre Patria” (Spain) and the former colonies are strong.
In the same way that Americans and Britons sound so different, so do South Americans compared to their Spanish counterparts. The very recognisable “c” sound, such as in “Barcelona” is non-existent in all South America, where the “c” in Barcelona and the “s” in “Salamanca” sound more or less the same.
However, variations in accent and vocabulary are significant within South America. Although most speakers would understand each other, Colombians, for example, would say “pimiento” and Uruguayans would say “morrón” (pepper). Differences exist even across neighbours: while Uruguayans say “ómnibus” for a bus, Argentineans say “colectivo”. And don’t even ask South Americans what they call a car! Anything from carro to coche to auto and more!
The way of addressing people also varies. River Plate speakers (Argentina and Uruguay) are famous in South America for using “vos”, a very casual version of “tú” (“you”) as well as “usted” (more formal). Each way of saying “you” (vos, tu, usted), will have variable grammatical combinations and must be used in the appropriate context. However, for example, Colombians use “usted” in more familiar/informal speech.
Brazil, as you probably know, is a former Portuguese colony and therefore speak a different language. However, Brazilian Portuguese is understood fairly well by South Americans, particularly by those in neighbouring countries. “Portuñol” (the Spanish-Portuguese blend, similar to Spanglish in the US) is fairly common around the Brazilian borders. Then we also have the small yet unique Suriname (official language is Dutch), Guyana (official language here is English) and the French Guyana (and you’ve guessed, French being the official language).
The good thing is that most South Americans, whatever their background, will be appreciative of English speakers making the effort to speak Spanish (or Portuguese), and will be very relaxed about mistakes. Most will be very keen to try their own knowledge of English – but please do not make the assumption and do impress your South American business counterparts with at least some basic knowledge of the language! It shows you really care.