Lumi

Once the magic returned and people began transforming new forms of communication became necessary. One of the first challenges faced was that of the merfolk. While they retain the capacity to speak above the water, they are unable to speak when submerged. At first they attempted to use sign language but that was challenging for the folk with webbed fingers. It was also challenging to speak at depth.
It was quickly discovered that most of the merfolk had the ability to control some form of bio-luminescence. For some the eyes glowed, others had patches along their tails. Those that did not possess it naturally developed spells which would work in a similar fashion and spread that knowledge among the others. With that as a foundation they began to develop both an alphabet and the equivalent of shorthand to communicate. Based on morse-code, as a set of dashes and dots, the language was quickly decided up and widely distributed. Unlike those on land, this one language was universally accepted. This is considered a dynamic language with words and phrases constantly being added and modified based on learning to live underwater, as well as those who have been born underwater and lived their whole lives there.
A robust documentation of the language insures that those who need to communicate with merfolk can, with the added benefit that the language can be adapted and learned as necessary. They have their own news broadcasts that have a word or phrase of the day so that the language can stay aligned despite the distance between pods. It also allows them access to the latest news and information from the land.
The return of Atlantis did introduced another lexicon, but once informed that their other brethren were using the merlingo, they quickly adapted and utilized it as well in order to maintain a unified language for all aquatic dwellers. Atlantis did bring in a few new words and phrases since they had generally adapted to aquatic life for a longer period of time.