Ill. Admin. Code tit. 56 § 5220.800 : — LABOR AND EMPLOYMENTJOINT RULES OF THE HUMAN RIGHTS COMMISSION AND THE DEPARTMENT OF HUMAN RIGHTS: NATIONAL ORIGIN DISCRIMINATION IN EMPLOYMENT — Speak English Only Rules

a) An employer may not have a rule requiring that employees speak only in English at all or at certain times where the rule has the purpose or effect of discriminating against individuals on the basis of national origin. It is a defense to an allegation that a "speak English" rule has a disparate impact on a particular national origin group to show that the rule is justified by business necessity.
b) It is common for an individual whose primary language is not English to inadvertently change from speaking English to speaking his or her primary language. Therefore, if an employer believes it has a business necessity for a speak-English-only rule at certain times, the employer should inform its employees of the general circumstances when speaking only in English is required and of the consequences of violating the rule. If an employer fails to effectively notify its employees of the rule and makes an adverse employment decision against an individual based on a violation of the rule, the Human Rights Commission ("the Commission") and the Department will consider the employer's application of the rule as evidence of discrimination on the basis of national origin.

Ill. Admin. Code tit. 56, § 5220.800