What did you say?
Language is an important part of culture, but it is also a terribly impractical thing in a game. In the real world we have things like the tribe in one valley speaks an entirely different language to the tribe in the next, but in a game world, while entirely realistic this is just annoying, and often easily circumvented by the PCs (Comprehend Language for example).
So how do you justify having a “common” language that is readily understood by most if not all the world’s inhabitants? Well the answer I think is Empire, and 4E handily comes to the party with this in its core concepts and Nerath.
You take an Empire that entirely dominated the region the adventures take place in and have the Empire have a single language – cf the British Empire as a real world example. This then gives you that basis for a common language.
Racial languages then become more important in establishing the culture of those races and their identities as ‘not belonging’ to the empire. Use of the common language is a matter of practicality from when your neighbour was larger than you and learning their language let you prosper from trading with them. Even if you have the Empire be monolithic and say that it once ruled all the lands racial enclaves would still exist where the racial language would be used – either openly or secretly.
Settings like Rokugan exploit this idea, everyone in the Empire must speak the Empire’s language or else they cannot communicate with millions of people. In DnD the idea of the fallen Empire is rampant, Forgotten Realms is the only major DnD setting that lacks a unifying Empire (yes I know there have been Empires in the Realms but none have unified enough of the land mass under them to establish a truly common language that would persist into the modern era).
So why do we care about this?
Well our real world experience tells us that people in different lands speak different languages, and we are often left with the question of “what is common?” as knowledgeable players question why everyone speaks the same language.
Determining the ‘why’ of common helps explain the ‘how’ of the setting, because language is such a key component of our ability to prosper, both through trade of goods, and more importantly the retention and exchange of knowledge.
Taking the 4E default setting you immediately get some obvious hooks. The common language of the PCs is likely closely related to that of Nerath, and the existence of long-lived races like elves and dwarves helps reduce the language drift. So we now have a reason why ruins that are hundreds of years old have a language in them that modern PCs can still understand. Maybe it is a bit like reading Shakespeare today, but you can get the meaning of it.
Then there are the really old ruins from Bael Turath and Arkhosia, they can have their own languages. In fact them having different languages is important as a means of differentiating the two cultures and their ruins. Now Arkhosia seems to be a prime candidate for having Draconic as it’s language, another language that would have limited drift (as even in times of extinction from a region dragons being very long-lived will keep it alive and relatively unchanged). In comparison Bael Turath should have its own language, while some might have spoken Supernal much of the Empire’s existence would be based on a more mundane existence and thus language. Now Turathian isn’t in the PHB so it may be a matter of practicality to have Supernal act as the language in ruins from the Tiefling Empire. Now this means characters have a reason for learning Draconic, and Supernal later in a campaign if those languages are important for understanding what is going on in the ruins your bad guys are using as a base (or if you do time travel etc). Or if nothing else those languages are now more a part if the setting as characters use Comprehend Languages to understand those ancient scripts.
(Primordial serves a similar role in looking at even older ruins as well.)
This brings me to a lot if the point of this, in most DnD games it is my experience that languages are seen as “modern” much like people see languages today in the real world. As such because they often exist separately to a culture they are not something that helps build a world and create links, and thus immersion. By using languages in specific ways and having people react to common for its association with the last Empire (or the Dragon Kings or whatever caused its adoption) that gives it depth and stops it just being another thing on a character sheet.