Accessing Rare Languages (2024 rules)

I think the argument that the rules are meant to be setting agnostic becomes null and void, now that all of the 5.5E expanded rules have options during character creation. Thayan? Netherese? Even in a very Forgotten Realms setting - these are not common languages. 

As a DM, I would much rather have all of the languages listed during Character Creation and separated as "Common Languages" and "Rare Languages" with the caveat note to the player, "Rare languages are rare for a reason. We would recommend checking with your DM prior to selecting one of these languages."

The reason I say this, is because during creation, characters are limited to a specific number of languages deliberately. Having to manually add it after the fact either adds an additional language to their character sheet, or leaves part of the character builder unfulfilled with that annoying blue box. Pedantic, and a little OCD yeah... but I have plenty of private convos with my players before and after session zero where we hash out character choices, and I would much rather have it an option for them instead of a homebrew step for me.

You are entirely correct, and you aren't missing any hidden rules exception. Under the 2024 Player's Handbook character creation rules, language selection is streamlined so that every character automatically gets Common plus two choices strictly from the Standard Languages table. Because iconic planar and underground tongues like Infernal, Celestial, Abyssal, and Undercommon are officially classified as Rare Languages, starting characters are walled off from taking them at level one.

This creates a glaring thematic disconnect for species that were previously defined or flavored by these tongues. Under the new guidelines, an Aasimar or Tiefling has no rules-as-written way to organically start knowing Celestial or Infernal, despite their literal celestial or fiendish heritage. Unless a specific class feature explicitly grants access to rare languages later on—such as a Ranger's Deft Explorer or a Rogue's Thieves' Cant—characters are completely locked out of speaking their own traditional heritage languages at creation.

The design intent behind this shift was likely to decouple language from species to allow for more diverse backstories—explaining that an Elf raised by humans wouldn't automatically know Elvish. However, locking out otherworldly and regional dialects entirely from character creation creates an unnecessary hurdle for narrative archetypes, like a Fiend Warlock or a Deep Speech-touched Sorcerer. Most tables will likely bypass this oversight with DM permission, but strictly by the book, it is a surprisingly rigid limitation.