Manual:$wgDefaultLanguageVariant

Details
Default variant code, if false, the default will be the language code.

Only works when it is defined as a variant in the  class of that language.

Usage
In LocalSettings.php, one may set:

= "sr"; $wgDefaultLanguageVariant = "sr-el";

This will use Serbian system messages in Latin-script, but report for javascript the simple contentlanguage="sr".