- How to turn it on? It works on enwiki, but doesn't seem to work on
srwiki (f.e. try searching for итацизам that exists in srwiktionary). Doesn't seem to work on enwiktionary even...
If everything goes well, it'll be enabled on all projects this weekend.
- Could it also display sister project results if only a few matches exist
in current project?
yes, even when there is no match in local wiki, one could get matches from sister projects.
- Could it include sister projects in other languages, of course if all
language projects agree? Obvious application of this are b/h/sh/s wikis, but also recently announced Egyptian and classical Arabic, and similar cases.
yes, one can group wikis together in pretty much arbitrary way. The only limitation is that once wikis are grouped together, all of them get results from the same group. I grouped them into sister projects because that seemed most relevant. The only exception currently is meta-mediawiki.org group. As for ex-yu languages, I'm not sure if there would be consensus for such a feature, but if it were, it could be enabled.
r.