I have added a small script at http://www.toolserver.org/~platonides/activeusers/activeusers.php to show active users per project and language. Requisites for appearing there are more than 500 edits (total) and at least one action (usually an edit) in the last month (since May 16, data is cached). Bots appear in the list. I'm still populating the data, but it should be completed by the time you read this.
I have done the intersection part http://toolserver.org/~platonides/activeusers/intersection.php
I find the results to be quite useless for the original goal. Almost all entries are bots. Even intersecting big wikis like de-en [1] or en-es [2], where many people is able to speak both languages, only shows one user. So my conclusion is that people stays on its home wiki, and it is very strange that someone passes 500 edits *both* on its wiki and in a foreign one.
For the record, going through the whole list to get the active users took 30m26.207s. Not bad for 797 wikis. Actually doing the intersection took 3m47.916s. The app doesn't check sul accounts, instead it naively takes equal usernames as being the same person. All wikis were compared for actions after 20110516081337. The drift between that epoch and the point where the query was done was not compensated.
1-http://wolfsbane.toolserver.org/~platonides/activeusers/intersection.php?pro... 2-http://wolfsbane.toolserver.org/~platonides/activeusers/intersection.php?pro...