This is a really simple extension that allows semi-arbitrary links to be appended to the "Languages" section of a page's sidebar. It can be locked down so that (for example) extra language links can only be added to the Main Page (T18962),
or so that extra language links may only point to Old Wikisource (T34189) (no longer needed due to the introduction of $wgExtraInterlanguageLinkPrefixes in core).
It's only about 200 lines of code all up, so it shouldn't take very long to review.
Here is "Greg's checklist" for this extension:
Extension page on mediawiki.org: Yes
Bugzilla Phabricator component: Yes (was requested in T66784)
Extension in Gerrit: Yes
Repo creation requested
Design Review: No, but since it is so simple, I doubt it really needs one
Architecture/Performance Review: T66903
Security Review: T66902
Community support: Not needed in the case of T18962 (ELL offers a more robust implementation of an existing "feature")
; seems unnecessary in the case of T34189 (simple uncontroversial enhancement request)