Brazil makes up 51% of the land and population in South America, making Portuguese the dominant language on the continent.
Spanish, which is the official language in all countries except Brazil, Guyana, Suriname and French Guiana, is widely spoken and is useful even in the countries that are not historically Spanish speaking.
If your stay will include Brazil and border regions, a Portuguese phrase book will be extremely helpful. Portuguese and Spanish are to a very small extent mutually intelligible, and written forms will be understood, though with difficulty, with speakers of the other language.
In Guyana, Suriname and French Guiana, the official languages are English, Dutch and French respectively.