Manual:$wgExtraLanguageNames

Language, regional and character encoding settings: $wgExtraLanguageNames
List of language names or overrides for default names in Names.php.
Introduced in version:1.12.0 (r28212)
Removed in version:still in use
Allowed values:(array of strings)
Default value:[]

DetailsEdit

List of language names or overrides for default names in /languages/data/Names.php.

$wgExtraLanguageNames['xyz'] = 'Language name';

NotesEdit

You may need to also add the prefix to $wgExtraInterlanguageLinkPrefixes for it to actually behave as an interlanguage link when defined.