If the language you are interested in isn't in the top 10, you can still find out how popular it is on GitHub. Just click on your language in the list under "All Languages" on the right side of the page. You will discover, for instance, that "FORTRAN is the #32 most popular language on GitHub".
Does anyone know how this "most popular languages" list is calculated? I can imagine several methods:
* Number of projects containing a language.
* Number of projects having most of the code in a language.
* Lines of code in a language.
* Number of users having a language in some repository.
Plus there is this whole forking thing going on...