The languages/i18n/sco.json exists since April, 1 2014 and we even have a wiki https://sco.wikipedia.org/ without the corresponding MessagesSco.php.
At least the following should be initially added:
$namespaceNames = [ NS_MEDIA => 'Media', NS_SPECIAL => 'Special', NS_MAIN => '', NS_TALK => 'Talk', NS_USER => 'User', NS_USER_TALK => 'User_talk', # NS_PROJECT set by $wgMetaNamespace NS_PROJECT_TALK => '$1_talk', NS_FILE => 'File', NS_FILE_TALK => 'File_talk', NS_MEDIAWIKI => 'MediaWiki', NS_MEDIAWIKI_TALK => 'MediaWiki_talk', NS_TEMPLATE => 'Template', NS_TEMPLATE_TALK => 'Template_talk', NS_HELP => 'Help', NS_HELP_TALK => 'Help_talk', NS_CATEGORY => 'Category', NS_CATEGORY_TALK => 'Category_talk', ];
Also:
/** Only if the namespaces do vary between genders */ $namespaceGenderAliases = [ NS_USER => [ 'male' => '', 'female' => '' ], NS_USER_TALK => [ 'male' => '', 'female' => '' ], ];
You could also add any other stuff that is listed at MessagesEn.php should you wish to also translate aliases, special page names, etc. You could see MessagesEs.php as an example or whichever other file you'd like to use.
@jrbs You have self-identified as a speaker with proficiency of Scots. Could you please at least try to translate the $namespaceNames or ask somebody with knowledge of Scots to provide them to us?
Thank you!