And if it’s in the extension, it could use PHP instead of JavaScript so that the links appear immediately on page load. Although I don’t know how much control extensions have over the language link list’s appearance.

And yet it’s still used on a bunch of wikis…

In T211314#9849978, @Tacsipacsi wrote:

And yet it’s still used on a bunch of wikis…

I believe that code is broken and does not do anything.

Some copies (like the Czech or German one) are completely broken and don’t do anything other than consuming bandwidth and CPU unnecessarily (they assume that HTML classes are listed in some given order). Others, like the Romanian, Hungarian or the oldwikisource one (the latter is linked in the task description) technically work: they add the ⇔ links to the language links, which add the ?match= parameters to the URLs – but those URLs don’t work anymore, since the extension disappeared, resulting in a confusing user experience. (Examples: Romanian, Hungarian, Danish, which imports the oldwikisource version.)

Both versions are bad and should have been cleaned up by a global interface editor when the extension was undeployed over half a year ago.

I've removed everything that was Wikisource-related :)