I would generally argue that original D&D was built on two pillars - combat and exploration. Social interaction and negotiation mainly functioned as a tool of exploration, rather than being a pillar in their own right.
Absolutely, but I'm not arguing that social interaction was a pillar in its own right, just that early editions of D&D had an expectation that players would try to avoid combat, and on of the ways of doing that was via negotiation, and therefore languages had more importance in those editions, giving us the holdover of language skills today.