Which "local languages" are top priority and why?
This is on many levels a nonsensical question as we do not invest in languages. When we invest in strategies that help small wikipedias, they will help any and all small wikipedias. You do not have to concentrate on that.
More relevant is that certain topics are not properly supported. Africa is underdeveloped for all its subjects even in the English language Wikipedia. Cultural bias like this makes Wikipedia less relevant to the people in Africa. At that the English language Wikipedia has the most impact on the dissimulation of information about "Africa". I expect that with improved coverage more intrest will be generate for local African languages.
As this moment the WMF gives priority to the top 50 languages in terms of numbers of speakers.