But translatewiki.net can also help translate things like donation banners or policy pages, right? So, why not properties on Wikidata? I think that would be great! But it's more like translating page content, not system messages.
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