My response is not directly related to editor contributions to Japanese Wikipedia but you may still be interested to know:
We are aiming to learn more about Japanese Wikipedia readers (aim = we're interested, we asked a member of the Japanese Wikipedia community to help us translate the survey and other documentation to Japanese). You can follow the task at
https://phabricator.wikimedia.org/T151835 This is a follow up on
https://meta.wikimedia.org/wiki/Research:Characterizing_Wikipedia_Reader_Behaviour to assess whether the results of that study holds true across languages. The other languages we're considering are Spanish, Hindi, and maybe Arabic (the list of languages is not fixed at this moment, but we do have translations for Spanish and Hindi ready as well).
Anyhow, follow up the task if you're interested to keep track of it. :)