You probably want either the getFormattedNsText or getNsText method of the language object.
It depends a bit what language you want it in. If you want to use the page language [e.g. what's stored in page_lang field in db, falling back to ContentHandler default, and then falling back to Content language], you might do something like
$article->getTitle()->getPageLanguage()->getFormattedNsText( NS_MEDIAWIKI );. [Where $article would be the first argument to the ArticleViewHeader hook] If at all possible, this is probably the version you want. If page language isn't what language the page is actually written in, you probably want to fix that rather then use a different language object.
If you want to use the content language (Default language of the wiki):
MediaWiki\MediaWikiServices::getInstance()->getContentLanguage()->getFormattedNsText( NS_MEDIAWIKI );
If you want the user language, use
$article->getContext()->getLanguage()->getFormattedNsText( NS_MEDIAWIKI );. Be very careful with this version though, as if it gets cached you may have cache pollution issues where users get the wrong langauge.
If you want to do it for an arbitrary language (replacing 'bg' with the lang code):
MediaWiki\MediaWikiServices::getInstance()->getLanguageFactory()->getLanguage( 'bg' )->getFormattedNsText( NS_MEDIAWIKI );