Misanthrope Prime
Adventurer
I think what would make sense, ironically going back to the days of OD&D with its "alignment" languages, is to have "planar language families"; we already have that with Primordial, where the various elemental languages are mutually intelligible to anyone who speaks Primordial.i'd like something like this if we're trying to get languages to matter, consolidate most languages down into say, six-seven 'core language groups' plus common, and then common doesn't get bonuses to social interactions, it's this weird hodgepodge language that near everyone knows but if they have a choice don't communicate in,
alternately, i'd be interested in exploring languages as based more as representing social group dialects, 'thieves', 'noble' or 'merchant' aren't really different languages from common but the word pool, terminology, sentence structure and mannerisms+etiquette of speaking all add up and it more represents a familiarity with the norms of the social group so if your character doesn't know how to speak one then there's a very large amount of subtext happening under the surface and they're missing as much as the dwarven speaker in a crowd all speaking elvish.
What I'd do is take the languages of some powerful beings- celestial, sylvan, infernal, etc- and pull them up and make them the "proto-indo-european" of the various mortal races. Elven and gnomish are part of the Slyvan family, dwarven and giant are related to primordial, etc. These languages being planar also explains why an elf from two different worlds can understand eachother.
We know common is a linguistic isolate unique to Sigil, though perhaps one could consider undercommon a creole language of common and drow-elven.