Note the language breakdown, under the Graphs tab.
But yes, it all hinges on how Github measures popularity. For one, I'm curious how forked repositories are counted; if I have a 100kloc Blub project and it gets forked 100 times, does Github count that as 100,000 or 10,000,000 lines of Blub?
defunkt has confirmed that at least in the past if you had a repo with 100 Perl files and one JavaScript file it would count equally towards both languages.