The dealership cannot require that non-Spanish-speaking customers only work with Spanish-speaking salespeople: doing so could be national-origin-based discrimination against the non-Spanish salespeople. (And if it is an African American salesperson who has customers taken from him, could be seen as racial discrimination, too.)You can, of course, make the customer aware that you have Spanish-speaking salespeople and ask the customer if he/she wants one of them to assist them: that is not discrimination, since you are not requiring the switch from one employee to another.