It is currently not possible to manage interwiki links to/from multilingual Wikisource (https://wikisource.org) on Wikidata. We want to make it possible to connect pages there with the other language-specific versions of Wikisource so these interwiki links don't have to be maintained in the wiki text anymore.
[ ] it is possible to add sitelinks to multilingual Wikisource in Items on Wikidata
[ ] After adding a sitelink, the sitelink appears on the Item page (in the Sitelinks section), in the API response (JSON, RDF)
* We accept the inconvenience of not being able to link to several pages from the same wiki in the same Item.
* Sitelinks should show up with language code mul in the sitelinks section of an Item.
* Sitelinks should show up in the Wikisource sitelink section of an Item.
** As noted in T204706#4707035 it might be that due to current logic baked into Wikibase code, the sitelink will not be out-of-the-box. If this occurs, the desired section of the sitelinks section is to be agreed on with @Lydia_Pintscher
* The process of adding such a "special" wiki to Wikidata should be written down for further use in similar cases