Normally, it was a dead language in the North Thorass language group. Basically, this Auld Tharian Language predecessor for the Tharian tongue, which has been spoken in the Northern Moonsea region. You can check this languages attributes from below lines.
Attributes Of Auld Tharian Language
Language Family: Faerûnian languages
Spoken by: Humans
Language Subgroup: North Thorass
Based on: Italic
Language Group: Thorass