This is a planned variable, see bug 11267, to be used in LocalSettings.php setting maximum number of fallback languages per user and defaulting to 2.
Setting this value to 0, or false, disallows users to specify individual fallback languages in their preferences. This was the standard up to MediaWiki 1.14.
Values of 1 or above allow users to specify up to $wgUserFallbackLanguages fallback languages in their preferences, which will be used for interface messages not present in the localization selected per either the
uselang= parameter on the command line, or the user language in the preferences, before the fallback languages of the individual localizations are used, each in the given order.