The Common Tongue
Why does everyone speak the same language? And if everyone knows the common tongue, why do we not always speak it?
The common tongue is the language of gods and spirits. To speak it is to tresspass. Not in a serious way, bringing down divine vengeance. It's more like when you are invited to some sort of event where you're an outsider, where any moment you'll make a mis-step, a breach of etiquette, and be found out.
Nobody quite speaks the common tongue fluently, without years of study and solitary contemplation. It's the language used to create the world. Wizards use it, they even think they can use it properly, and that's why wizards are so dangerous.
It is like a language of your childhood. You can understand it when you hear people speak, but the words are clumsy on your tongue and you feel ashamed to have forgotten.
It is like the dialect your grandfather spoke before it was declared that it is a thing of the past. And now you know it is too late to properly learn it, you'll only ever know the modern new replacement, the language of your ancestors will always be foreign to you.
It is like your name when you have lived too long in a foreign land and have forgotten the correct way to pronounce it. It's the most familiar thing there is, but you have to stop and concentrate and shape your mouth just right and sometimes it isn't quite there.
Every mortal understands it, and every beast. Even the rocks and the trees understand it, perhaps, though they rarely rouse from slumber.
When given the chance, most people prefer to speak another language, a language that is easier, that comes more naturally, that is shaped for mortal lips and tongues. A language for swearing and for commanding, for telling jokes and writing poetry and falling in love. Not one that reminds you that this is not your world, that you are just a guest here for the limited span of your life.
Mechanics:
You can use the common tongue to speak to any creature capable of speech, including supernatural creatures. If a spell allows you to talk to plants or inanimate objects, they will speak in the common tongue. When you can communicate with someone in a language other than the common tongue, any action that involves building a rapport, communicating complex concepts, or otherwise speaking skillfully, gains a mechanical advantage appropriate to the game system.
Header image in the public domain, adapted from here