The DoubleWiki extension displays an article and its translation on two columns of the same page. It was written specifically for Wikisource.
The extension is enabled on all subdomains. To see it, add '?match=xx' to the url of a page, where 'xx' is the language code of an existing interlanguage link.
In wikis that have configured it, you may click on the "⇔" symbols near interlanguage links. In order to enable these symbols in your subdomain, copy the BilingualLink function from fr:Mediawiki:Monobook.js to your wiki. (you must be a sysop to do that)
Visit any of these pages, and click on the "⇔" symbols near interlanguage links: