A person can partially swallow there tongue but since the tongue is attached inside the mouth there's no way for a person to fully swallow there tongue and have it end up in their stomach.
The human mouth has a small piece of tissue called the frenulum linguae, which sits behind the teeth and under the tongue which holds the tongue in place.
So even in a seizure a person cannot fully swallow their tongue even though it may seem like the person has swallowed their tongue it's impossible to fully swallow the tongue.