Everyone with a pilot's licence has to pass a checkride that includes emergency simulation, use of checlists, and use of the radio. Periodic medicals also look for problems that prohibit pilots from operating aircraft. Seems to me the rule you mentioned already exists, hidden in the certification and recertification process.
As far as professionalism goes, I'd rather have professional coworkers that had a speech problem than ones who clearly spoke words of intolerance.
Matthew.