Feature summary

In T275958: Enable interlanguage links from multilingual Wikisource to other Wikisources, interlanguage links from multilingual Wikisource to monolingual Wikisources were enabled. However, they are not shown as interlanguage links the other way round, i.e. from monolingual Wikisources to multilingual Wikisource. They do appear in the In other projects sidebar section, but that’s not shown on mobile, and multilingual Wikisource is not really another project. They can be added to the interlanguage links section in wikitext with [[mul:…]] syntax (see the More languages link on s:pl:Autor:Tomasz Zan), so it should be possible to add them in Wikibase code as well.

Benefits

• Multilingual Wikisource becomes more reachable on mobile (and also on desktop using new Vector: the other projects section is hidden in the hamburger menu, interlanguage links are much more prominent in the upper right corner).
• Consistent interface: if I reached a monolingual Wikisource from multilingual Wikisource through an interlanguage link, I want to get back the same way.