He asks her to say things in her language, and write the things she's said, and he tries to repeat them and change them around some, and this can in fact take quite a while. Harry seems to like his job.
There will be food available when they need food.
There's a bit of a talent for languages there, probably from knowing seven (and a half, depending on if runes count) reasonably well already, but she won't be writing the Great American Novel any time soon.
Most of the notes will be sent to a computer anyway to create something that can automatically translate stuff (and be turned off when Mountain wants to speak English and practice, naturally).
This language apparently has lots of room for syntactic ambiguity. She writes two sometimes quite different versions of most sentences longer than five words. (Or maybe she's just not all that good at English yet.)
"English feels like four different languages smushed together... Valecana," (the language he's currently in the process of decoding,) "hates loanwords."
"Yes, but Valecana is new as languages go, a few hundred years ago King Lica invented it and insisted all his ministers learn it. It was popular with everyone else because it was the 'Royal Tongue' and a few years later he had schools teach only in Valecana. Krellian gave us a few words like gata" (sickness) "But mostly just stopped existing."
"Royals of Graya are close to all-powerful, and even then it took a long time for it to stick. It probably wouldn't have if he wasn't also regarded as an administrative genius."
"I suspect what I call technical and what you do are very different. I tried making some of the technology we use in my world and it didn't work."
"Indeed. Though I want to take a break and start on my mountain, or build a road or fix a bridge or something, soon."
"I'm going to make a mountain in the middle of the ocean. Besides just wanting to, I need to rest in a place that is 'mine' to get back to full power."
"Alexandria told me she'd find a place. I was going to just go to... I think it was the 'Catskill Mountains,' but apparently they're public property. No merfolk in this world, though, so: The middle of the ocean."
Harry goes to the door and knocks on it, and it's opened from the outside. He talks to someone there and returns. "They'll give you a waterproof interactive map to show you a good location."