A salary differential of $42.50 biweekly shall be paid incumbents or positions requiring bilingual proficiency as designated by the appointing authority and Human Resources Director. Said differential shall be prorated for employees working less than full-time or who are in an unpaid leave of absence status for a portion of any given pay period. Designation of positions for which bilingual proficiency is required is the sole prerogative of the County and the decision of the Human Resources Director is final. The Association shall be notified when such designations are made.