What would happen if an American girl spent her time living with, speaking with, and learning slang from some girls from the north of England? The answer is the same: depends on the girl! Some Americans in Britain go 'hyper-American' and protest that everything American is better; some do the opposite and end up with hilariously fake 'English' accents; and some walk the middle path and just sort of absorb things.
So you might end up with someone like Agent Huinesoron, who's disdainful about mortals and thinks in Quenya (deliberately so). But... okay, let's design a character.
Lethril ('eavesdropper', hilariously - and yes, there's a masculine form as well!) is a fairly quiet Green-elf from the First Age. She lives in Doriath, speaks Doriathrin Sindarin, and can vaguely write - she's aware of the runes, knows about half of them, that sort of thing. (No, the Doriathrim weren't particularly literate! See P@L's comments on 'Sindarin-style record keeping'.) Her mother is a hunter, her father a smith. Her favourite season is winter, because the ice on the river reminds her of crystals; her most treasured possession is a dwarf-made bracelet set with faceted quartz crystals.
(As an aside: see how easy it is to invoke a character without spending your time listing powers and weapons?)
And then she's dumped into OFUM and has to learn English, and is abused by the staff over her habit of telling her friends back home whimsical stories about life around Cuivienen (she doesn't understand why - what was wrong with her tale of Bainion the Fair and the way he healed the rift between the Lindar and the Gódhellim by being Just Plain Better than everyone else?).
She'd likely pick up English very quickly - elves do, you know. Given that she's a native Sindarin speaker, her main problem might well be a tendency to soften the first letters of words - 'the 'irls vrought me flowers, and I was hincerely voved' - but she'd get a handle on that. Conversely, when switching back to Sindarin, she might lose the mutations, which would sound equally strange.
She'd probably use colloquial English - she's picking it up from conversation, not from lessons, after all - but that wouldn't filter back into Sindarin. She'd probably start mixing idioms between the two, though - the Doriathrim were great mixers of culture.
Her outlook on life... there's a lovely speech in the Leithian Script, where Luthien thinks about her association with the mortal Beren:
Beren, you've made me see time as a mortal woman does. It's been an hour already! How will I survive a day -- a week -- a year?
And that goes for Lethril as well. When all your companions are getting enthusiastic about the passing of a week, it's impossible to keep the mindset that anything less than ten years is a blink of an eye. So yes, Lethril will end up thinking in 'mortal time'.
Another probability (also from the Leithian Script) is that she'll become somewhat addicted to mortals. It's their worldview! Lethril is 254 years old; she's seen virtually everything already, it's all a bit samey. But the girls at OFUM, they're seeing things for the first time. Lethril would never have looked at Legolas and thought 'wow, he's hot' - she knows his grandfather, he's of her people, she's spent her life around people just as pretty as him, her built-in assumptions mean she doesn't bother to look - but now that her friends say it, she's looking, and she's liking what she sees. ^^ And not just his hair, if you know what I mean.
Would Lethril come out of it with PTSD? Um, possibly from the teachers! If anything, she'd end up with Stockholm Syndrome - her family back home would hardly recognise her!
Lethril is fairly heavily based on Finrod Felagund's mindset, I think. He ran into the Edain and shacked up with them for a while, and you can totally imagine him dressing in rough furs and getting into a drinking contest with Beor. But you could equally make her a Thranduil, isolationist and withdrawn - or a Caranthir. Pick a model and project them into that situation; if you know the character, you can work out their reactions from first principles.
hS
PS: And yes, she'd love the stuffed toys. Menegroth was sculpted as a 'forest underground' - stuffed animals are a way to do that in every room! What's not to like? ^~