Definition from Nolo’s Plain-English Law Dictionary
A workplace requirement that prohibits employees from speaking any language other than English. An English-only rule is valid only if it is justified by business necessity and limited in scope to serve those business needs. For example, if an employer adopted an English-only policy for workers on a factory line for safety purposes, that would be justified by business necessity. If the same employer prohibited employees from speaking any other language while eating lunch or smoking in the company parking lot, that would probably be too broad.