How to set the languages to show in different countries

I have a multilingual site, Spanish being the main language and English the second one.

I want all Latin America, Spain and Portugal to get the site in Spanish. I want the rest of the world to get the website in English.

So far I know that people in USA and UK are seeing the site in English, but all the rest of the world see the Spainsh version.

I have read a lot around on this but I cannot find an proper explanation on how to do it. Any help will be highly appreciated.