It depends with the terms that the restaurant has in place. The restaurant should notify you when you go for the interview. However, it would be best to take out the piercing in order to improve the chances of getting the job.
Probably because your tongue ring could possibly fall out of your mouth into someones food (if its a fast food restaurant like taco bell where you can see the person making your taco or whatever) they can sew the restaurant and you would get fired... I guess