By the DM, 2012-10-16
All versions of the D&D role-playing game, and many of its d20 spin-offs, have a language called “common,” which all humans know how to speak, along with other humanoids and even creatures native to other planes, like elementals. It's relatively consistent over time, and everyone, everywhere, keeps up with changes in the language. This is convenient for gaming purposes, because otherwise, even the least intelligent character would have to know dozens of languages to survive, but it doesn't make a lot of sense. There are hundreds of languages on our world, for a single species. Why just one in D&D land?
In a world where one powerful Human kingdom dominates a huge region, political and military leaders can force their language upon others. There are limits to this explanation though. Does the kingdom dominate the whole world? Does it influence other planes, forcing extraplanar beings to learn Common?
With magic like Teleport and Sending around, human language just didn't diverge as much as it did on Earth. Easy long-distance communication means less variation. But what about isolated lands and tribes? Even if a wizard teleports to your kingdom once a year for some political negotiations, that doesn't mean you'd take on his language. And what about other races? What has compelled all elves to learn Common as a second language?
In order for the magic explanation to work, you need a relatively unified, human-dominated world, and powerful magic has to have been commonplace for a very long time. Interaction with other planes must be easy.
Common is just the simplest language to learn. Humans find it very difficult to pronounce all the complex sounds in Elven, or rumble gutterally like an Earth Elemental, while other species see learning Common as child's play.
Common isn't the only language of Humans. Maybe every kingdom has its own language, but Common is a trade language that everyone knows, which includes words from many languages. This simplified speech is very easy for other races to learn, so anyone who wants to interact with humans at all learns it.
Any other ideas? Mention them in the discussion below.