ActiveUsers  is a tool written by Duesentrieb for finding users that were active in the last minutes/hours, filtered by groups, tags or categories. See User:Duesentrieb/Tools for other tools I wrote.
The ActiveUsers tool can for example be used to find an active wiki admin speaking a given language (indicated by babel boxes). In fact, for doing this on commons.wikimedia.org, there is a special mode with a simplified input form and terse output.
Note: this tool uses the Atom feed provided by the RecentChanges page, and caches it for 5 minutes. If the feed is found in cache, the tool should respond within a couple of seconds. If a fresh feed needs to be loaded, this may take longer (up to a minute or so).
Also, note that the tool always looks at the last 2000 edits, regardless of the time span (this is a limitation of the atom feed). In busy times on a large wiki, it may cover less than an hour. On a small wiki, it may cover several days.