I'm sorry but according to @C933103 (Wikidata:Project chat#Adding label in any arbitary language.), it's still not supported in labels, descriptions, and aliases. By typing mnc in Special:SetLabelDescriptionAliases it still says The language code "mnc" is unknown. Please use a language code known to the system, such as "en".
Labels/descriptions/aliases and monolingual text deliberately do not use the same sets of languages, so a code added for monolingual text is not automatically available for labels.
This request specifically asks for it for monolingual text. If someone wants it to be available for labels too, according to the recent RFC about that, they need to get it added to MediaWiki (see https://translatewiki.net/wiki/FAQ#How_to_add_a_new_language)