Reasonably well traveled American here. 10 countries over 5 continents as of this comment.
You should go and see the world. It's an amazing experience! I think wanting to learn more languages is commendable and I hope you do. But even if you don't, don't let that stop you from traveling. English is widely spoken, especially in the service industry. And translation apps, while clunky, work well enough.
Your Grandpa doesn't sound like he wants his mind changed. I say that partly from what you said, and partly because I know the type. And it's hard to convince anyone who isn't interested in changing their preconceived notions. Which is sad, being stuck in a box of his own making. I can understand you wanting him to see it the way you do. But if you can't, that's not your fault. And he can't hold you back unless you let him.
I share your sentiment about MAGA people in general. But in this situation you're also saying it to a 15 year old kid about their Grandpa. I think it's a situation that calls for more tact.