Your typical old-school D&D setting is basically a howling wilderness with only small bastions of civilization, right? Basically a post-apocalyptic world. (It certainly explains all of the dungeons and other ruins.) And yet, the people in these isolated fortresses and Medieval-Europe-Meets-Mad Max-style ramshackle villages, who have little contact with outsiders, little in the way of wealth or resources, and little access to what we would consider "education" in the modern sense, all manage to speak at least one common, consistent language. And if I'm not mistaken, many (even most) of these people are generally portrayed as literate, too! We're talking modern or near-modern literacy levels in a setting that is both pseudo-medieval and post-apocalyptic. That's got to be some kind of miracle, right? Some kind of magic?
Well, why not explain all this by sticking a Tower of Unity somewhere on your world map? A reverse-Tower of Babel, the construction of which created a common tongue. The aforementioned blog post called it the "Lexicos Spire." You could even go a step farther and say the Tower somehow creates or enforces other commonalities between the far-flung remnants of civilization, like a common form of currency and a common set of by-laws for all those Thieves' Guilds and Druidic Circles and such.
And maybe a well-organized coalition of bad guys are trying to bring down the Tower and pull a Babel on the world for their own nefarious reasons. Since this would make life a lot more difficult for the PCs, they might want to do something about it.