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 LanguageConverter class of that language.

Usage
In LocalSettings.php, one may set: $wgLanguageCode = "sr"; $wgDefaultLanguageVariant = "sr-el"; This will use Serbian system messages in Latin-script, but report for javascript the simple contentlanguage="sr".