Hustrilian

"Oldcommon", usually called Hustrilian by basically everyone except Hustrilians, is, despite it's name, a very rare and widely unknown ethnic language specific to the Hustrilian people. The language is also, unsurprisingly, nothing like Common, with completely separate grammar and sentence structuring. The only thing it shares with Common (and the reason people call it Oldcommon) is because the Hustrilian alphabet was used for the Common language, as it was the most simple and easiest to teach young children.