The ISO code for multi-lingual resources is <code>mul</code> and this was recently added to the interwiki map to allow for links such as <code>[[mul:s:Main Page|OldWikisource Main Page]]</code> but there's no mul.wikisource.org.
Again, note that mul.ws *does* exist now--thanks for that. Now, we should make that a functioning subdomain so we can have a clean landing pad just like at wikibooks.org or wiktionary.org, etc. <code>mul</code> is a valid ISO code and should be the subdomain. All incoming links to wikisource.org (e.g. https://wikisource.org/wiki/User:Koavf or https://www.wikisource.org/wiki/User:Koavf) should redirect to the mul.ws equivalent (i.e. https://mul.wikisource.org/wiki/User:Koavf) except for Main Page and a handful of other names that would be an index.