Actually those language names are translated (as "nan" and "rup") in all CLDR languages, except their own because they're not CLDR language codes (yet). I have a patch in the works to pick up the translations which use a different code than MediaWiki's, but I guess you want to add the "autonym" i.e. the name of the language in the same language, right? The ideal way to do that is adding the locale to CLDR, you see at CLDR how.
Actually I just hope that we can have a translation in every language for the language name of "zh-min-nan" (cf. the interlanguage links' tooltips of https://fr.wiktionary.org/wiki/Wiktionnaire:Page_d%E2%80%99accueil?uselang=fr : the second to last link is for "zh-min-nan", and the tooltip displays the English language name "Chinese (Min Nan)", instead of the French one.
I guess your patch will solve this issue, if I'm not mistaken.
Unfortunatelly, I doubt if CLDR can have time to handle all languages of the world, as Nikki said in phab:T151269: The CLDR officially rejected the addition of Kven (fkv) and Pite Sami (sje), and in an early request they also vetoed (?) the addition of Buryat (bxr)
and Moldovan (mo).
So far, we should still encourage users to request addition on CLDR, but if they refused such codes, isn't it not bad to add such translations of codes to https://phabricator.wikimedia.org/diffusion/ECLD/browse/master/LocalNames/LocalNamesEn.php , https://phabricator.wikimedia.org/diffusion/OMWC/browse/master/wmf-config/InitialiseSettings.php , and something in https://phabricator.wikimedia.org/diffusion/ECLD/ ?