I am intrigued by the idea as well.
Yes, it is not system messages of the wikibase extension, it is
system
messages of a wikibase instance as Denny correctly explains. But
then,
sharing theses translations in a central place would tremendously
help
those who need to set up their own wikibase installation. I believe
in
the longer perspective, a large percentage of the properties (the
descriptive vocabularly of "field names" in a database sense) that
separate wikibase installations would use, would consist of re-use.
Whether the re-use mechanism involves translatewiki, or whether it
should become a function of wikibase is an open question. I think not
now, not while resources for the wikidata project are scarce, but I
think it should be conceptually on the map.
--gregor
Can actually translatewiki be merged into Wikidata?
Cheers
Yaroslav