While inferring the language of a web page can be difficult (Bonjour!), HTML is there to help. Look for the lang
attribute:
<html lang="en-us">
It should be noted that any element can have said attribute. In the case of my opening sentence:
<p lang="en-us">While inferring the language of a web page can be difficult <span lang="fr">(Bonjour!)</span></p>
More info here: https://stackoverflow.com/a/7076990/1216976
Alternatively, you could check the Content-Language
of the return headers, but that's not as specific, defining the entire page.