Note: this seems to be about a cross-wiki search, rather than a cross-language search on a multilingual wiki. The current search should already search content in all languages in the current wiki.

Is there a way to cross-link Wikisource pages across editions, similar to how Wikipedia links articles? Does that even make sense?

Is there a way to cross-link Wikisource pages across editions

There is more than one way. The traditional way is to have local interlanguage links https://en.wikisource.org/wiki/Wikisource:Interlanguage_links (interwikis) but for a few years now we've had a mixed model with Wikidata/Wikibase and various core/template/gadget/extension trickeries. That's by itself a large discussion best continued elsewhere; T138332 and https://www.wikidata.org/wiki/Wikidata_talk:WikiProject_Books have some part of it.

Insofar this task is a request for a cross-wiki search to appear on Special:Search, engineering-wise this is probably easiest to provide in CirrusSearch with something like the "Results from sister projects" box enabled by $wgCirrusSearchEnableCrossProjectSearch that you can see e.g. on https://fr.wikibooks.org/?search=Dante . There are going to be limitations to that, for instance if the index for all Wikisources together is unwieldy.

A "proper" advanced search across all languages might one day be possible through Wikidata, or through something like Structured Commons, but we're far from having all the information available in a structured format.