My grandmother has dementia and has recently been hospitalized. Every time my parents visit her, they encourage me to TAG ALONG. I NEVER go. I love her AND ALL,

Sure, sure, you love her and ALL. What is it, may I ask, that encompasses the "AND ALL" portion of that statement?

But she doesn't remember that we've been to visit her.

How the hell do you know what the crazy old bat remembers? Just because she doesn't remember when someone asks if you've visited, or doesn't tell of the visit to others is not a guarantee that she doesn't remember or realize it during her quiet time alone.

She left me some money, and my dad tries using that to make me feel guilty when I don't go with them.

Gee, I guess guilt is all your dad has left, since apparently appealing to your common decency isn't working. His own fault, he should have been able to teach you some kindness by now. Give the money to charity if you really want to circumvent that guilt. Even if you don't have it yet, you can pledge it. Dare you.

To me, her body may still be there,

So why not pay attention to her body then? Give her a hand massage. Brush her hair. Paint her nails (use water in a nail polish bottle) Bring her the gift of a scarf and arrange it in her hair, on her head. It's all about the touch of a hand. Does your grandmothers hospital allow pet visits? Perhaps bringing an animal with you will enliven the interaction for you.

but her mind is pretty much gone.

I hope you're pretty much positive of that. I dunno, maybe you are a child genius neurosurgeon, who is making breakthrough discoveries about the brain, and you are positive... if so please let me know.

When I used to go visit her, she often started conversations in HER NATIVE LANGUAGE.

What language? Do you speak any of it? How's about learning a phrase or two, lazy ass? Maybe someday you can go visit her relatives in her homeland...

She has even confused me with HER NIECE that passed away.

That niece is was a relative of yours too, genius. Does grandma see something of her in you in some way, I wonder how she died? Something genetic? Something scandalous? Do you know?

I can't talk to my dad about this because it's his mom and he's really sensitive about it.

Yes because everybody knows it's best to ignore the issue of people's moms dying. If you don't mention it, you're dad will forget all about his grief.

I feel I'm too young to be dealing with something like this,

All of us are always to young to be dealing with something like this, but if we are lucky, we all do. Yes lucky, because the only other option is a child dying before their parents and grandparents - and no no no!

so how do I talk to my dad about it without him becoming upset?

You can't. But ignoring the issue, and blowing off your dad when he needs his family around him most, in not a better route to take. Your father needs you now, go visit only to support him. It's reason enough, and if you can't manage to make the sacrifice of discomfort for him, then well, it doesn't speak well of your character, you selfish little twit.

FWIW - your letter is a great example of excuses and red herrings, kudos for that, you have a great future in avoidance ahead of you.