I have heard this will kill them, is it true?

Yes

yep

And very quickly. Caught one on a 6xd that was dead before I got it in the boat. Does anyone know why it kills them?

Yes, from my experience

Yep!

In a heartbeat it will! And they wont even bleed from it.

Lots of blood vessels in the tongue. TPWD did a study where more than 50% of fish hooked in the tongue or throat (esophagus) died. Only 25% hooked in the gills died. Dunno if that was gill cartilage or gill rakers. Every fish I've ever hooked in the gill rakers that was bleeding, died.

Depends where in the tongue but usually you will know pretty quick. Gill hooked fish sometimes make it if you have treated water. I've had bleeders make it all day. I think the catch and release treatment stops minor bleeding

Yes Had a 4 1/2 die in a tournament cost me second place for dead fish penalty. It was fine when I first put it in there and dead 30 mins later when I checked on it.

I have a 13# fish on my wall that still has a 5/0 Owner buried in it's tongue. Never have I seen a fish die that quickly. When I got her to my taxidermist and showed him the hook, he told me that the main artery that brings oxygenated blood from the gills to the heart runs right at the base of the tongue and my hook had severed that artery. I had intended to release her, dropped her in the livewell which was running with cool water (10:15 pm in March) got mt camera ready and when I opened the livewell a couple minutes later the water was re and she was floating belly up.

First time I saw this I was mad because he chunked back about 5 pounder in a tournament first thing that morning. not even five minutes later the same fish floated up dead! Didn't bleed or anything just hooked through the tongue.

If you got hooked in the tongue wouldn't you want to die? Just figure out how those with all the tongue piercings survive though!

So obviously in a tourney, if you tongue hooked a fish you wouldn't want to keep it because of the dead fish penalty and such, but given the facts that they are pretty much going to die, wouldn't it be considered wasteful since you could at least eat it later?
If it is a legal keeper in a non-tourney situation would it not sway you to just keep the fish for the fryer later? I don't always keep bass when fishing but there are times that I will and one of those is if I thought the fish was going to die anyway (gill hooked, bleeding profusely, hooked in tongue etc.).

I was showed something years ago where a guy I was fishing with caught a fish in the tongue and immediately rubbed some of the catch and release on the tongue. The fish survived, do not know if it was a fluke or not.