Most of the people that are in the Coastlands come from other places, specifically other planes. They speak a variety of languages of their home planes, but rarely have any understanding of any of the languages in use in the Coastlands. So most people that arrive here will initially be unable to communicate with anyone and will appear to be speaking gibberish to anyone hearing them. They will hear things that Coastlands inhabitants say as gibberish as well.
To get around this difficulty, several wizards in Oldtown have worked together to create a potion that grants sentient beings the ability to understand what they are saying or hearing as a single common language. Because of the large numbers of people that arrive in the Coastlands through the various gates, they produce the potion in large batches and distribute it throughout the city and surrounding countryside regularly. The potion is distributed as single doses in blue-tinted vials capped with corks. It is readily available in most taverns, inns, and common stores in the area. The usual price ranges from 1-10 common gold pieces.
Once a person has imbibed the potion, they will feel a burning sensation in their mouth and throat. Their ears will pop, and they will have a slight headache. These symptoms will pass in a couple of minutes, and the imbiber will be able to understand and speak the Common tongue of the Coastlands. They will still retain the ability to speak any other languages their character knew before imbibing the potion, including the Common tongue of whatever realm they came to the Coastlands from, but they will need to find others that can understand them to communicate effectively. Long-term residents of the Coastlands occasionally lose the ability to speak their original language due to disuse.