Wikisources pages are connected using sitelinks to the edition items of Wikidata. So, different editions of the same work on different Wikisources will not have sitelinks to the same items, breaking the automated language links feature.
We should add a feature that uses P629 (edition of) to find if two Wikisource pages shares the same work and so build interlanguage links between them.
A nice way to retrieve these links would be to do a SPARQL query or to use P629 to get the work item and P747 (edition) to get all the editions.
It could be implemented inside of the "Wikisource" extension that is not yet deployed or inside of the WikimediaBadges extension that already deals with the other projects sidebar customization for Commons