RG,

Yes, the tongue is part of the mouth.

Here's my interpretation:

I believe the "hooked in the gill, eye, or tongue..." line in the regs refers to the fact that a fish hooked in those areas is usually mortally wounded, and will probably die. The "where legal to do so" means that if the area is open to retention, and the fish is hooked in these areas you should keep it. If you're fishing in an area, or a time of year that does not allow any retention, then the fish must be released no matter if it might die or not. For clarification, "gills" means hooked FROM THE INSIDE, not hooked from the outside in the gillplate.

I'm pretty sure that this applies to all areas, not just ones with special regs, unless specifically stated.