The official language of Illinois is English. Nearly 80% of the population speak English natively, and most others speak it fluently as a second language. The forms of American English spoken in Illinois range from Inland Northern near Chicago and the northern part of the state, to Midland and Southern dialects further downstate. Illinois has speakers of many other languages, of which Spanish is by far the most widespread. Illinois's indigenous languages disappeared when the Indian population was deported under the policy of Indian Removal.
English is Illinois's official language. Illinois was one of the first states in America to pass an official language law, though its first official language was not English but "American". In 1923, Representative Washington J. McCormick of Montana had failed to pass a bill in the United States Congress declaring "American" to be the official language of the United States. Following the bill's failure, Senator Frank Ryan introduced a similar bill to the Illinois General Assembly. The bill passed with the support of Irish and Jewish politicians in Chicago, who, by rejecting the term "English", wanted to show their opposition to British policies in Ireland and Palestine, respectively.  In 1969, another act of the General Assembly replaced "American" with "English." Meanwhile, the term "American language" survives in some legislation from the period.  Unlike official language laws in many other states, the statute in Illinois is purely symbolic, having the same status as laws naming state symbols like the state bird and state fossil. 
The English of Illinois varies from Inland Northern in the northern part of the state, to Midland and Southern further south. The Northern Cities Vowel Shift is advanced in Chicago and its vicinity, and some features of the shift can be heard along The St. Louis Corridor, a southwestern extension of the NCVS stretching from the Chicago area to St. Louis.  The distribution of dialect features reflects the origins of early English-speaking settlers: Inland Northern predominates in areas first settled by people from New England, New York, Pennsylvania, and Germany; Midland and Southern are spoken in areas settled by people from the southern United States.  African American Vernacular English is also spoken in parts of the state.
As of 2010, 78.26% (9,315,206) of Illinois residents age 5 and older spoke English at home as a primary language, while 12.74% (1,516,674) spoke Spanish, 1.64% (195,600) Polish, 0.67% (80,954) Chinese (which includes Cantonese and Mandarin), and 0.64% (75,754) Tagalog. In total, 21.74% (2,587,254) of Illinois's population age 5 and older spoke a mother language other than English.  Among those who speak a language other than English at home, most (57.3%) were reported in 2010 to speak English "very well." 20.7% spoke English "well," 15.9% spoke it "not well," and only 6.2% "not at all". 
|Language||Percentage of population|
(as of 2010) 
|Chinese (including Cantonese and Mandarin)||0.67%|
|German and Korean (tied)||0.41%|
|Urdu and Russian (tied)||0.35%|
|Greek and Hindi (tied)||0.30%|
In the colonial period, the Miami-Illinois language was spoken throughout the territory of the modern state. Jacques Gravier, a missionary at Starved Rock, wrote the first French-Illinois dictionary in 1700; Jean Le Boullenger wrote another in 1725.  Other indigenous languages at the time of statehood in 1818 included Potawatomi, Sauk-Fox, and Ojibwe. These languages disappeared from Illinois when the U.S. carried out Indian Removal, culminating in the Black Hawk War of 1832 and the Second Treaty of Chicago in 1833.
French was the language of colonial Illinois before 1763, and under British rule remained the most-spoken language in the main settlements of Cahokia and Kaskaskia. Many French speakers left Illinois after it came under U.S. control following the Illinois Campaign of 1778, but Illinois was still a bilingual territory for several more years before the flood of Anglo-American settlers overwhelmed the old communities.  By the time of statehood in 1818, French was in decline for matters of politics and government.  Colonial French died out in Illinois but survived as Missouri French in some communities across the Mississippi River.  The 2010 census reported 34,938 French speakers in Illinois, 0.29% of the population. 
- Baron, Dennis (n.d.). "Language Laws and Related Court Decisions". Dennis Baron: Professor of English and linguistics. University of Illinois at Urbana-Champaign Department of English. Retrieved 30 March 2014.
- For example, 225 ILCS 705/27.01
- Baron, Dennis (2001). "Language Legislation and Language Abuse: American Language Policy through the 1990s". In Dueñas González, Roseann; Melis, Ildikó. Language Ideologies: Critical Perspectives on the Official English Movement. 2: History, Theory, and Policy. Mahwah, New Jersey: Lawrence Erlbaum Associates. ISBN 0-8058-4054-0. Retrieved 30 March 2014.
- Friedman, Lauren (2012). "A Northern Cities Shift Retreat in the St. Louis Corridor" (PDF). 36th Annual Penn Linguistics Colloquium. University of Pennsylvania. Retrieved 16 February 2015.
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- Ryan, Camille (August 2013). "Language Use in the United States: American Community Survey Reports" (PDF). Census.gov. United States Census Bureau. Retrieved 30 March 2014.
- "The Illinois Identity: Language". MuseumLink Illinois. Illinois State Museum. 2000. Retrieved 30 March 2014.
- Boggess, Arthur Clinton (1908). The Settlement of Illinois 1778-1830 (PDF). Chicago: Chicago Historical Society. p. 50. Archived from the original (PDF) on 2010. Retrieved 30 March 2014.
- Boggess, 135
- O'Connor, Rorye (15 November 2012). "French still thrives in the Illinois territory". Mt. Vernon Register-News. Mt. Vernon, Illinois. Retrieved 30 March 2014.