You ask about accents and LotR... Consider: common as a second language to every member of the Fellowship except for the hobbits. Also (except for the hobbits) all being from wildly different cultures, idioms are lost in translation FAR more often than they are found.
Anon, how does it feel to be my new best friend
Common is Aragorn's third language, after Gondorian and Sindarin; he tends to switch from time to time and feels most comfortable in Sindarin, but some of the hobbits are personally betrayed that he's not actually really curt and blunt by nature, he just has less experience in Westron and would rather use simpler, more direct forms than risk mistranslating something. (Which is not to say he's particularly poetic in any of his other languages--Legolas's Westron structure would imply that Sindarin just has a more elaborate syntax--but a good deal of his affectionisms are in Sindarin for a reason, and that is because he has addressed royalty as the Sindarin equivalent of "dude" when he was still learning and is desperate to avoid a similar mix-up, in front of hobbits, no less)
Boromir also learned, very much book common and will occasionally slide into a Westorian pidgin; where Aragorn will usually describe around a word he doesn't have, Boromir is more likely to just casually ask "Is there a Common word for ŃĐ°ŃĐżŃŃĐžŃĐ°?" forgetting that the hobbits would have no possible way of answering that. Mordor forbid they need to use a word that describes something that only exists in Khuzdul/Sindarin/Gondorian, because if they have something close to that in Common, they're all fairly different in connotation. (Such as weltschmerz vs ennui) Do Merry and Pippin teach Boromir swears in Common? Absolutely.
Gimli is unfathomably amused by Westron-exclusive words but has trouble grasping the exact context. (Is a hobbit dreikÀsehoch--three cheeses high? The hobbits say it's usually reserved for wee ones, but then have no problem referring to a human child of the exact same height as dreikasehoch. Does a schnapsidee have to be a bad idea, necessarily?)
Sindarin has shrimp emotions, much like German; Legolas's Westron is probably a bit better than Boromir's or Aragorn's in structure, but he does have a tendency to assume that if he can't think of a direct word for the situation, Westron just doesn't have it. (Legolas, facing down a battalion: "Well, this is a bit of a pickle") In Elvish culture, it's generally considered more polite to act reserved whenever possible, but Legolas's Westron has a very hobbit-y cadence to certain words. Sindarin is also a language with accents more related to structure than pronunciation that has to be pronounced exactly as is and nobody bothers to tell Legolas that Westron is accented significantly because every so often he'll use a colloquial term which has a thick brogue or accent and Aragorn quietly loses it.
Gandalf, as @penny-anna mentioned in this post, is the only one who can consistently understand everyone.
As for idioms and colloquialisms:
The Sindarin speakers try their best to translate Elvish phrases as literally as possible if they're not 100% certain what the same sentiment in Common is. They really do. But not only does a fair bit of it relate directly to a play on words, rhyme, or pun, but also makes absolutely no sense to someone who's not super familiar with thinking in Sindarin.
("Do you intend to sing the stars from the sky, little bird?" asks Legolas, amused, referring to downplaying a great task until it sounds manageable and in reference to a faerie tale of a rooster who vanquished the first Great Eagle's talons from the moon with his proud song)
("Legolas, what the heck are you talking about" says Frodo)
Dwarvish expressions tend to be almost exclusively related to splendors and glooms and just a. A bit overbearing.
("Have you broken the bones in your nose?" asks Gimli cheerfully, referring to the equivalent of "Has the cat got your tongue?")
("Um. Not yet" says Frodo.)
However, there are some delightfully silly ones that overlap quite well with Westron, such as the equivalent of à stÄ med skjegget i postkassa--to stand with one's beard in the mailbox--meaning, to get caught in a bit of a mess that's exclusively a consequence of one's own fault.
Gondor idioms tend to be notoriously difficult to translate. Boromir uses an expression like "raining cats and dogs" or "table it for now" which he....tries to explain, but even taken as literally as possible, doesn't make nearly as much sense.
("Heard it straight from the horse's mouth" says Boromir, referring to something he heard from the original source.)
(Sam glares cautiously at Bill the Pony)
But hobbit idioms--hobbit idioms are a thing to be cherished because there are so many and they're all outright ridiculous, and too often animal or food themed.
("Say, I could go for a pint of dwarven courage, could'na you?" says Merry playfully, assuming it translates the same to Khudzul. Gimli scoffs at first, but laughs near to tears when he hears that it refers to alcohol.)
("That one's a hard nut to crack." says Pippin, in response to a riddle.)
("Well, this is a fine kettle of fish" says Sam, referring to a situation that is, in fact, not nearly as pleasant as one would expect a hobbit to respond to a kettle of fish)
While a good bit of the alcohol-related ones are at least understood in sentiment by the rest, the Westron and Sindarin speakers have come to many a playful debate about the proper roles of animals in expressions--is a fox or snake more clever and conniving? Is a donkey more stubborn, or reliable? Are rabbits better known for moving quickly or not showing up at all?
Eventually hobbits become known in the Fellowship's expression as quick, greedy, playful little monsters, about the same role as foxes in English idioms, pigs in German, and rabbits in French. Not one of them can complain about this.
