It isn't really possible for a person to swallow their own tongue. A small fold of tissue called the frenulum linguae holds the tongue in place even during a seizure.
Contrary to popular belief, forcing a foreign object into a victim's mouth during a seizure to prevent tongue-swallowing is both counterproductive and potentially harmful, and it is not advocated by health experts. Even if the tongue flops back and blocks the airway during a seizure, the restriction is only temporary and corrects itself when the seizure runs its course. It is far more likely that a seizure victim might choke on saliva or vomit after breathing resumes, so the best course of action is to turn the seizing person on his side to keep the airway clear.