Alright everyone, I believe I have discovered what is causing the issue with blank Human names, or at least something that fixes the issue.

The TL;DR of it that the PLAINS entity (human civ) does not have any [SELECT_SYMBOL:REMAINING:] symbols, and giving it some seems to fix the issue.

More in depth:
So I ran into the same issue with names not generating for humans. My first assumption was there was some typo in the language_HUMAN.txt file of the mod, so I did a quick juryrig test of just replacing the language_HUMAN.txt file with a copy of language_DWARF.txt since those names seem to be fine. This does not fix the issue.
So I checked the other language_SYM.txt and language_words.txt to try to figure out how languages actually work and stuff, and I learned that nothing race/civ specific shows up there. I then went to check another mod that affects naming, specifically 'Slightly Less Urist.' This mod does not add any new vocabulary. All it does is give [SELECT_SYMBOL:REMAINING:] symbols to the MOUNTAIN entity (Dwarf civ). So this clued me in that symbols are what is used for naming, and moreover naming is done at the ENTITY/civ level. So I go and investigate the vanilla ENTITY raws. As far as I was aware the blank name issue only affected humans, and Dwarfs, Elves, and Goblins were fine, so I just needed to compare the symbol tags of the PLAIN entity to the other 3 entities. As as it turns out, The vanilla PLAIN entity does not have [SELECT_SYMBOL:REMAINING:] tags while the other 3 entities do.
So I have no idea what REMAINING is in regard to the [SELECT_SYMBOL] tag, but given its naming I assume it is a catch all for naming other things not covered by a specific [SELECT_SYMBOL] tag (such as [SELECT_SYMBOL:WAR:] for names of wars), and that includes names for creatures apart of that civ. I have no idea how or why the human civ generates names fine with just vanilla language and entities files, so it might just be something like the hardcoded game data is doing something on top of what the editable raws are doing for human civs, like having hardcoded symbol names. Either way adding some [SELECT_SYMBOL:REMAINING:] tags to the PLAINS entity does fix the issue as far as the world legends of a couple worlds I generated show. I simply just copied Slightly Less Urist mod and replaced MOUNTAIN with PLAINS for the small entity change it does. I have not bothered to check if this results in humans with just names from the added symbols or if it includes the names of normal humans, but what is important is that no blank names generate.