The Common tongue

[Datt]

We use common as a "trade language". Meaning that you can say things like "how much" and "do you have", but you can't carry on a conversation. Needless to say my dwarf didn't understand the rest of the party for a while until he decided to finally learn Chondathan, the most spoke language of the group. It was real fun when we got blasted over to Mulhorand and NOBODY spoke Mulhorandi. Luckily there were a few who understood dwarven and I was able to find someone so that I could translate with the party.

 

[Zander]

The Common Tongue is, as others have made clear, just a handwave to remove the impact of languages in play...

And in literature. Tolkien was the first to discuss the concept of a "Common" language in a fantasy setting AFAIK (though not the first to use one). You can blame/credit Prof. T! If he hadn't used "Common" in Middle Earth, we may never have had it in D&D.