"As elsewhere in the USA, English is the major labguage spoken . ... Midwestern accents tend to be a bit flat with just a touch of nasal twang, but compared to other parts of the USA, most of the English you'll hear is pretty standard - the middle of the country has always produced a share of of plain-spoken TV announcers. ...."
Thus being written on page 12 of my recently purchased Chicago city guide (Lonely Planet, Melbourne/Oakland/London/Paris, 2004).
I hope, the other infos in the guide are correct