One of the weakness points observed in 20200225-mediawiki interface language incident is there was no inline documentation associated with the variable which could have prevented the incident.
The setting was restored in change 574743 and associated task T246071 was subsequently closed as resolved, but no documentation was added.
This point of weakness is clearly actionable and so should be acted upon. Another safeguard mentioned on Gerrit is to move the setting to CommonSettings.php since the value affects all wikis and looks unlikely to change.