(I have not read through the 15 pages - so what I say here may have been already said, ro argued away. If this doesn't add anything for you, my apologies. I just wanted to get my initial reaction to the OP down...)
Eh. I don't think racial languages were the foundational principle. The relevant foundational principle was using "race" as a (problematic, easily criticized) proxy for culture.
As I recall the maps of old, D&D used to have regions (usually broken up by terrain type) controlled by races. Here there be giants. There's the hobgoblin nation. Lizardmen fill these swamps. Dwarves in these mountains. Elves in these forests. Orcs roam these plains and hills, and so forth. In this model, race/culture, and the associated languages were still associated with regions.
The problem came when we discovered that, really, players love playing different races, but they do not love playing the impacts of racism or being cultural outsiders. I can hardly blame us, 'cuz racism sucks more than D&D languages do.
The end result is that our worlds on large and small scales tend to have permanent cosmopolitan racial diversity, but still assume race=culture for linguistic matters.