The multilingual Wikisource (a.k.a. OldWikisource) is one of the few wikis that really needs this function. Please enable it so that we can mark pages by language there.
Mentioned in SAL (#wikimedia-operations) [2017-09-13T00:11:25Z] <thcipriani@tin> Synchronized wmf-config/InitialiseSettings.php: SWAT: [[gerrit:377357|Enable Special:PageLanguage on mul.wikisource]] [[gerrit:377674|Fix correct wiki name for wgPageLanguageUseDB on www.wikisource.org]] T175622 (duration: 00m 49s)
Both things requested above: mark the default language as <code>mul</code> and make it so that individual pages can be edited. See e.g. https://wikisource.org/w/index.php?title=Index:Festival_Romanistica.pdf&action=info and https://outreach.wikimedia.org/w/index.php?title=Main_Page&action=info (the latter is editable and the former is not).
Since the translate extension was enabled (T201814), which includes this feature by default, I believe this can be closed. I don't think setting the default page language to mul is possible without changing the wiki language itself, and that would probably have really bad side-effects, so it's probably better to keep it at English. But please feel free to reopen (and possibly change the task description/title) if I'm wrong.