Common is a language that is proposed in the rulebooks as a universal and, as its name suggests, the most common language in the world the game takes place in.
While every Dungeon Master has its own opinion about Common, certain solutions and explanations are encountered more often than the others:
- Common is the only language in the world. This is the easiest way to deal with it, especially if neither you nor your players are linguists and want to dive into the differences in Dwarven and Elven idioms.
- Common is the only language in the part of world used for playing. It can be safely assumed that there are indeed other languages, but they are encountered only far from the main civilisation: deep in the dwarven mountains the group of characters can meet an old sage who does not speak Common, or a horde of barbarians can have difficulty with it, otherwise everyone uses it. When using real historical settings, one can say that Common in French (if you play French musketeers), Spanish (if you play pirates of Spanish Main), or whatnot.
- Common is the language used for communication. This is the role English language plays now in the real world: most of the educated people in civilised countries can speak at least some English, no matter what is their native language.
- Common is the pidgin language used for trade. If this is the case, not everything can be said in Common. You can certainly explain that you want to buy that sword for 100 gp, but convincing a drunken suicidal gnome in the fact that his life still has some meaning might be a big problem. Your DM may not let you express some words when your character is speaking Common, or may ask you to roll a skill or ability check.
- There is no Common. Some DMs prefer to get rid of Common, because it confuses them and their players.
While there are no strict rules about Common, everyone is free to make their decisions.