The release languages (and work lyrics languages, since it is the same list) are indeed ISO based. Most of the languages are actually in the database but hidden since they would clutter the list and have never been used - when a user requests one, we unhide it. Interslavic, since it was recognized in ISO so recently, was an exception and was not in our database at all yet, I added it now (and set the release as Interslavic).
That said, the question was about why a language is not in the aliases options. The aliases locales are locales, not languages nor scripts, and they follow DateTime::Locale - Localization support for DateTime.pm - metacpan.org which itself follows Unicode CLDR. That list is annoyingly restrictive (new locales are added by Unicode only if someone proposes them and champions them in some ways including adding a bunch of data and translations in that locale IIRC). We do update our locale list every once in a while (we are on CLDR 47 and 48 is now available so we maybe should update) but it will always be a lot smaller than the language list as long as we stick with CLDR.