Hi everyone!
I’m happy to announce that, according to [1], the support of Wikisource site links will be rolled out in Wikidata the January 13th, 2014 if nothing goes wrong.
After this date we will be able to add links to Wikisource in Wikidata items and, so, manage interlanguages links between Wikisource directly in Wikidata as it is currently done in Wikipedia and Wikivoyage.
The support of access of items content in templates (a.k.a. phase 2) will be rolled out later.
There is a page on Wikidata for coordination that contains also a proposal on how we should link Wikisource pages to current items: https://www.wikidata.org/wiki/Wikidata:Wikisource
Feel free to translate this message in your scriptoriums and update the coordination page accordingly,
Thomas (User:Tpt)