I'ld say mostly it is skewed towards languages that are commonly taught/learned as a first language or are hyped on the internet as the default answer to a certain requirement, while language that just trundle on quitely in industry use are highly under-represented. The former will generate the most questions and they will have the most projects on github.
The real observation we can take from this IMO is that there is a strong tendency for github projects to be written in "cool" languages like julia, rust, go, coffeescript or TeX relative to SO, while SO questions a relatively much more likely to be about "boring", "corporate" and closed source/proprietary languages. Thus tex and viml are now officially hipster languages, while using sql, xml, cuda, delphi or mathematice means you are quite clearly a corporate sell-out.
Now obviously we can't tell whether that relative difference is caused on either end or both but if I were to hazard a completely unscientific guess, I'ld say that github users probably self-select strongly towards being users of "cool" languages, because they want to show their cool projects to other people, while SO is a godsend for people who are forced to use a certain legacy technology and just want to get results.