Yes but it’s not as easy as you may think.
Because just giving someone that much knowledge….Dipper can’t do that. He can’t give that to the kids for a few pieces of candy. There is going to have to be something taken in return.
I see the kids asking Dipper this when they are thirteen or fourteen and Dipper hedges and says he’s going to need to talk with their parents first.
So after a bit Henry and Mabel and Dipper sit down and tell the triplets yes you can do this…but only for one language because you’re going to have to give up something big to gain this knowledge.
(Acacia went sheet white and Dipper smiled a bit sadly. He patted his oldest niece’s hand. “Nothing crazy kid, just like…um, giving up chocolate for a year. Sacrifice, dear.”)
Having freaked out the kids sufficiently, they have them further think about it for a few months before going through with it.
(Henry and Mabel argued back and forth a bit on if they’d allow the triplets to do this but Mabel won in the end. “Besides,” she told Henry. “It’s a good way to learn about sacrifice, and responsibility, and a whole new language! Henry still looked worried, so she rolled over on the bed to cuddle next to him. “Besides, this is Dipper we’re talking about. He’d never, ever, hurt them.” Henry relaxed slightly. “Point.”)
Eventually all three make their deals with Uncle Dipper: Acacia will not climb a tree for a whole year and in return learns Spanish, Hank gives up video games for the same period and learns Chinese, and Willow promises not to summon fire and learns Sumerian in return.
(her siblings looked at her. “What? It’s what all the older magical texts use. I can learn Spanish next year at school.”