California Code, Government Code - GOV § 12951

(a) It is an unlawful employment practice for an employer, as defined in subdivision (d) of Section 12926, to adopt or enforce a policy that limits or prohibits the use of any language in any workplace, unless both of the following conditions exist:

(b) For the purposes of this section, “business necessity” means an overriding legitimate business purpose such that the language restriction is necessary to the safe and efficient operation of the business, that the language restriction effectively fulfills the business purpose it is supposed to serve, and there is no alternative practice to the language restriction that would accomplish the business purpose equally well with a lesser discriminatory impact.