Radovarl said:
Campaign settings, however, really should strive to add a little more complexity to the situation, if only for cultural flavor. I mean, really, what is the chance that someone from Durpar (Forgotten Realms) speaks the same Common as someone from Waterdeep??? It's a pseudo-medieval setting, after all, so the sorts of mass-communication or travel technology that would permit consolidation of language are largely nonexistent.
Actually, somebody from Durpar would be speaking Durpari as their first language, and somebody from Waterdeep would be speaking Chondathan as their first language. Forgotten Realms does make a point that there are local languages not tied to a race or species that are widely used, and "common" is a trade language used as a second language. The Players Guide to Faerun even specifies that it's Player Characters that all known Common, not all characters. Your typical peasant in the Dalelands might speak only Chondathan, so if he ended up goibng north to the Moonsea area and tried to visit Phlan, the locals would be speaking Damaran, so unless he learns Common or Damaran he's going to be quite lost.
Being realistic, English really is the common language of Earth, it's not being Eurocentric or Ameri-centric, it's being realistic. Many good arguments have been put forth here for it. It might not be the first language of the majority of people, but it is a dominant language of trade, science, and diplomacy. If you were lost at some unknown place in the world, and you could speak either only Chinese or English, which would you choose? If it's Chinese, if you're in China you're in luck, but outside of there not so much, if it's English, you're likely that you'll run across somebody pretty quick who can at least understand you, it might not be their first language but it's understood.
I know if I was lost somewhere in the world, not knowing where I was, and I could put up a sign that said "Help" or a sign that had the Chinese symbols for "Help" or any other language, I'd go with English, because I believe that's the one that would most likely result in somebody realizing it was a distress call and sending help. Reasons like that are why it is the Common language.
It also depends on how you defiine "common", I see it as a widely used de-facto standard language that can be used at least crudely to allow people from widely scattered backgrounds to communicate. It might not be most people's first language, but it's something that most people can understand, at least a little, and if you're a world traveller it's probably the most important language to know.