By all means take this aspect of the job as a personal challenge and try it out. Remind yourself that being fluent is not the same as being able to communicate. I do know that some fast food restaurants have timers for their drive though personal; you need to have the person at the speaker at the payment window within a certain time frame. If this is the case in your workplace and you find yourself going over that time limit, you could ask your manager if they can make accomodations for you (in the U.S. workplaces must provide reasonable accommodations for workers with disabilities) that could lengthen the time you have to make orders or put you in a position where you don't have to work verbally within time constraints.