I don't doubt that the Github-based CMS will allow for community-provided translations. I _do_ doubt that we'll see anything like the current set of languages covered at anything like the current levels of breadth and quality.
Like I said in another comment, approximately nobody translates technical documentation for fun.
A surprising amount of mediawiki documentation is translated by volunteers (i cant speak to the quality as i dont speak nonenglish, realistically the english source material isnt that great) but volunteers do translate things
Notice that the English page seems obviously translated by a non-native speaker, which has some implications not favorable to Mozilla's behavior here, specifically that ESL people will contribute documentation in English that native English-language speakers won't even provide for themselves.
Like I said in another comment, approximately nobody translates technical documentation for fun.