I would recommend that you carry out five-year reviews of your employees' language skills as a means of quality control.
In an emergency, everybody works together to ensure that passengers are evacuated safely and in the best possible conditions, at least with regard to their physical well-being.
I appreciate that in the normal course of events, clients can receive service either in French or in English. However, in an emergency, if there are unilingual employees on the train, service is sometimes only provided in one language.