A dictionary of the extinct language of ancient Mesopotamia has been completed after 90 years of work.The Chicago Assyrian Dictionary is 21 volumes long, with many volumes dedicated to a single letter.It was put together by studying texts written on clay tablets discovered in the region, which is mostly modern-day Iraq.It has not been spoken for around 2000 years, and no one can say for sure exactly what it sounded like, but Dr Irving Finkel gave BBC World Service his best shot.