the pevensies return to professor kirk's house and tell him everything that happened, and it's only when they finish speaking that they realize they've spoken only in narnian the entire time. professor kirk can understand them, of course, but when they try to reach for the english words, all four siblings realize that they cannot remember the words they need.
so the months they spend with the professor are a mix of staying in their little beloved narnian bubble and recording all their memories and sharing stories, and also relearning english.
lucy and edmund are the two that have the hardest time relearning english. lucy, of course, because narnia is branded on her soul. she's aslan's dear one, speaking narnian is like breathing to her. edmund had to read and write countless contracts, letters, and treaties in narnian, so his brain is wired in the language.
all four pevensies have a little bit of a soft accent for the rest of their lives. no one can pinpoint what exactly the accent is, and it makes no sense to those who know them.
after peter and susan leave, and edmund and lucy are left with their aunt and uncle, they actually go back to speaking to one another in narnian almost exclusively, only using english around their aunt, uncle, and eustace. when they go to narnia with eustace, he doesn't start speaking the language until after he transforms back into a boy, then it's like he's spoken it his entire life.
when peter and susan visit with their parents, they walk in on lucy, edmund, and eustace having a debate on one of the old narnian stories about the golden age, and it's actually comforting for the older pevensies to slip into their main language.
after her siblings die, susan is terrified she's going to forget narnian, but she actually talks to herself in the language constantly, having conversations with the air, recounting memories, and whispering promises.