By default, a MediaWiki 3rd party install does not assume a multi-wiki multi-language setup. If there are interwiki links setup, they are probably to wikis of the same language.
After T326572 this setting would be moved to ULS.
In the mean time this should be false by default, and only enabled on WMF wikis.
Note also the config description is incomplete:
Moves the language links from the sidebar into a menu beside the page title. Also moves the indicators to the line below, next to the tagline (siteSub).
It also creates an "Add languages" call to action on pages without language links, which is why making this true by default more problematic than it was in Vector 2010.
Note that a wiki may install ULS for the interface language selector, but still not have any language links.