I think that's awfully sad! - But, to the original question, can someone tell me where the language code, whether en or mul/und part would appear at all? I don't understand what is being discussed. When I look at current interwiki links on WD, I see they have their language code next to it - surely the Multilingual Wikisource entries should not have "en" - is that the question here?

It doesn't appear anywhere on the Wikidata interface (AFAIK) but it is stored in the data and you can see it when doing a SPARQL query (like the one I gave before, more specifically with the schema:inLanguage predicate), probably in other tools reusing the data (through the API for instance).

Right now, (at least) 6 multilingual projects are tagged as in English: Commons, Meta, Species, Multilingual Wikisource, Mediawiki and Wikidata.

Cdlt,

~nicolas