Non-native English speaker here, I volunteer at several companies to translate their interfaces.
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.
The bar to entry to newcomers is probably more of a UX/usability problem than one of internationalization; that I agree with you on.
Translating the support docs is one suggestion I think is unequivocally good, and something that should be done.
I think internationalization is great in a read-only capacity, but when non-English speakers start posting Issues, comments and Pull Requests, that's where it starts to turn into a problem.
GitHub has very little to translate in the first place, as it's very sparse on prose, so aside from the question of support docs, we are probably making mountain out of mole hills, since the remaining English is jargon and not beholden to internationalization concerns.
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.