Wikidata is adding support for sitelinks to the Multilingual Wikisource.
There's a discussion there about which language should it use. Currently
it's "en", which is not quite correct: technically, the site language of
Multilingual Wikisource is English, but most of the content is in other
Ideally, each link should specify the language of the page to which it's
linking, but currently this is not possible technically.
Until it's possible, suggestions to use "und" or "mul" have been raised:
I don't have an opinion about this myself. Does anyone else have thoughts
Amir Elisha Aharoni · אָמִיר אֱלִישָׁע אַהֲרוֹנִי
“We're living in pieces,
I want to live in peace.” – T. Moore