Not so many years ago the only language that Americans needed was English. We are not close to other countries as they are in Europe and as Sunny points out, it can be a short drive in Europe to another country.
Canadians, until Quebec made French its official language in the late 1950s, spoke English. The influx of Spanish speaking North Americans into the USA has only recently become a large issue. This is, of course, do to illegal immigration from Mexico and a more mobile affluent population in Puerto Rico. After the Spanish American War in the late 1890s the US took protector-ship of the Philippines where Spanish is also the first language.
North America is a very large continent and unlike Europe has a large English speaking population. Until North Americans became more mobile and affluent, travelling to Europe was not common and so the ability to speak many languages was not a necessity. There was also, for a while, the Melting Pot theory that claimed that anyone coming to the US should "blend" into the existing society and become as American as possible, thus leaving their mother tongue behind.
I personally believe that every American school should now teach Spanish as a requirement as so many in the US now speak Spanish as a first language. Those who speak English as a first language should learn Spanish and those who speak Spanish as a first language should learn English. No exceptions.
While it would be nice to be multilingual and speak French and German (and in my husband's family Italian) these are languages that most North Americans don't need unless they plan to be in another country for an extended period of time. Then learning the language should be important and respectful of another county's citizens.
In the Western Hemisphere the main languages are English, Spanish and Portuguese and of course in Quebec - French. But the French spoken in Quebec is not the same French spoken in France.
The division of lands in South America can be attributed to Pope Alexander VI who decided to divide the lands there between Spain and Portugal.