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:[]

Details

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

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

Notes

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