Probably the language that is used the most on Shireon, Lenthiri is also most likely the most widespread constructed language in the world.
A Constructed Language
Lenthiri was originally made as a hobby project by the mage Lenthos. After working on the language for several years, he became so proud of his creation he decided it should be the main language used in his tower, and later the city that grew around it. As he was the one running the place and the one his students needed to listen to in order to even be allowed to learn the art of the Arcane, the language stuck and has been the official language of what later became the City of Lenthir for a bit more than a millennium.
A Language of Trading
As Lenthir grew and became the major trade hub on Shireon the language slowly got adopted as the international trading language. It's almost required by now to know Lenthiri if you're going to pursue a life as a professional trader. Only traders who just trade locally or in areas with generally similar languages (like the traders only doing business in Kupria and Vulborim) will have any chance of surviving as a trader without knowing Lenthiri, and even then it will be almost impossible to do trade with foreign traders visiting the area.