I have a list of country codes and a list of language codes. How do I map from country code to language code?

When the user visits the site, I can get their country code. I want to use this to set the default language (which they can later modify if necessary, just a general guess as to what language they might speak based on what country they are in).

Is there a definitive mapping from country codes to language codes that exists somewhere? I could not find it. I know that not everyone in a particular country speaks the same language, but I just need a general mapping, the user can select their language manually later.