Almost all people can read basic English (especially those who access Github), so translating the Github interface seems rather pointless to me. Of course, translating the support articles would really help accessibility for those who don't know English as well as native speakers.

Translating interfaces makes people think they can use their language to communicate on a site. Translating only the support articles helps people understand the site, but they will quickly realize that the site itself prefers English-only communication.