Hiding Behind the Language Barrier

Alice, Bob and Claire all speak Language A, which is usually the language the work is written in or translated into, and may be the "common" language of the world. But Alice and Bob also speak Language B, which may be their native language, while Claire does not, enabling them to have private conversations if Claire is in earshot or they know Claire is eavesdropping.

To qualify, Alice and Bob must also speak a language Claire can speak, and Claire's inability to understand the language must be her only obstacle to understanding it; if she's deaf and unable to read lips, it doesn't matter whether she can speak Alice and Bob's language.

This trope can also be used to keep information from the audience, depending on whether a translation is provided. If the language is real and not translated, it can be a Bilingual Bonus.

This trope can be subverted if, unbeknownst to Alice and Bob, Claire knows the language they are using. Such cases overlap with Bilingual Backfire. See also Censorship by Spelling which uses similar methods to achieve the same goal.

Fan Works

 * In A Complete Turnabout, Edgeworth and Franziska have a conversation about Franziska's past in German front of Phoenix, who only knows enough German to realize that Franziska does not like him eavesdropping.

Film

 * In Mission: Impossible - Ghost Protocol, a Russian character tells his wife in English that they need to get out of the country after the Kremlin bombing. His son, who's in the room, asks them why they're talking in English.
 * Attempted in The Lord of the Rings: The Two Towers, when Legolas tells Aragorn in Elvish that Helm's Deep's defenders are outnumbered and will die, but Aragorn angrily yells in Common that he will die along with them before storming off.
 * In Inglourious Basterds, the Jew Hunter speaks in English so the French-speaking Jews hiding under the floorboards can't understand what he and the farmer are talking about.
 * Oddly averted in Shanghai Knights. Chon Wang has a private conversation with his sister about Roy without knowing that Roy is eavesdropping. For some reason, they have the conversation in English rather than their native language, which Roy would not know.
 * In Braveheart, Lord Hamilton attempts this using Latin during the first conference between Wallace and the Princess, assuming a Scotsman wouldn't speak or understand Latin. It doesn't work, since Wallace speaks both Latin and French.
 * In the second Boondock Saints movie, Romeo speaks Spanish to his uncle to ask his uncle not to embarrass him in front of the MacManus brothers. He doesn't realize they know Spanish too.
 * In Close Encounters of the Third Kind, Neary complains about this when Lacombe speaks to Laughlin in French.
 * Pretty much the whole point of the movie Windtalkers.

Literature

 * Seen in Shanghai Girls. Pearl and her parents speak Sze Yp - a Chinese dialect, but her younger sister May does not speak this dialect. When Pearl and May end up in Chinatown in the US, this language barrier prevents May from talking to the family she has married into, and pushes her out into the broader world.
 * In Dune, the deadly Count Fenring and his wife make heavy use of Obfuscating Stupidity, including punctuating their speech with lots of drawn-out hmmmmmmmmmmmmmms as an apparent Verbal Tic. When the book switches to their point of view, it turns out the humming is a code language to let them privately converse and coordinate their act in full view of everyone.
 * One of the earliest scenes in The Night Circus is Hector and Alexander (presumably) issuing the challenge to pit Celia against one of Alexander's students... while Celia is standing right there. They pull this off through magic talk, but Celia cannot understand the adults while they converse and is oblivious to the topic of discussion.
 * In The Great Brain at the Academy, Tom is sent to a Catholic Boarding School for 7th grade (the town only has a 1-6 One Room Schoolhouse). Tom's brother Sewyn is there in the 8th grade class and takes delight in hazing Tom and the other 7th graders (as he was presumably hazed when he was in the 7th). At one point early in the book Sewyn exposits to another 8th grader, "Remember, speak in Latin when we don't want the 7th graders to know what we're saying." [Because the 8th graders had learned Latin when they were in 7th but the new 7th graders hadn't learned it yet.]
 * One of the CSI: NY tie in novels has a subversion, with Stella and Lindsay processing a scene at a bakery. The owner thinks he's getting away with calling them a derrogatory Italian term for a female cop, but Stella speaks a little Italian, and though she's rusty, she knows exactly what he's saying.

Live-Action TV

 * In the first season of 24, Jack and a police officer are pursuing a suspect, when they see a bystander and the officer yells in Spanish for the bystander to get down. The suspect takes the officer hostage, and Jack yells for her to fight back in Spanish. The suspect is captured but the officer dies in the process.
 * An episode of House has a Chinese mother and daughter talk in Mandarin so House won't understand. Of course, he speaks it. Another House example has House blackmailing a guy in Mandarin. Presumably, if he didn't get his money he'd repeat what he said in English.
 * In Frasier, Frasier and Niles have to speak French around Eddie the dog in order to not get him unnecessarily riled up over a potential walk.
 * In Revenge, Emily offers to act as a translator when Daniel is having dinner with a Japanese investor. The "investor" is actually Emily's mentor, and the two of them spend the dinner talking about Emily's revenge plots while Emily makes up translations for Daniel.
 * On How I Met Your Mother, Ted and Barney befriend a Russian bouncer who takes them to an underground poker game. Ted ends up winning a lot of money from the other players who are very shady and dangerous looking. Just as it looks like Ted is going to get killed, the bouncer intercedes, shouts a few phrases in Russian and things quickly cool down. Ted and Barney are grateful to have made such a great friend but do not realize that he actually told the other players that he plans to rob Ted and Barney and will return everyone's money when he is done.
 * Happens and also discussed in Northern Exposure. The older Native Americans all speak Tlingit when they don't want the youngsters to understand what they're saying. When Ed talks to Joel about this, Joel mentions a similar thing happening in Queens where Alter Kockers speak in Yiddish in order to keep secrets. Both Tlingit and Yiddish are dying languages; Ed decides to dub The Prisoner of Zenda into Tlingit in order to preserve the language.
 * Subverted on Scrubs. Turk learns Spanish, and listens in on a conversation Carla is having with her mother. He later tells her that he had learned Spanish.
 * On the first episode of Fringe, a couple of Iranian businessmen speak Farsi to decide on a figure to pay Peter Bishop. Unbeknownst to them, he also speaks Farsi, and agrees to the figure they choose.
 * In the Seinfeld episode "The Understudy", Elaine feels that her Korean manicurists make jokes about her when they speak to each other in Korean (she's right). Eventually, she brings along Frank Costanza with her, who speaks Korean, and gets upset when they insult him too.
 * In NCIS, Gibbs and Abby often speak to each other in sign language to avoid other people hearing their conversation.
 * On Impractical Jokers one time the mission was to get people to comment on bogus news stories in Times Square, and when approached two women commented "We don't speak English." It was lampshaded by the guy.

Newspaper Comics
"Foreign Speaker #1: Xmphlaca bi flucalaka un bijnana y aquaholder. Foreign Speaker #2: Gne!"
 * Dogbert's Clues for the Clueless gives insulting people in a foreign language they can't understand as an example of rude behavior (though not as rude as saying "ucket-bay ead-hay"):

Video Games
"Korialstrasz:Do they know, my queen? Alexstreasza: No, my beloved. (Draconic) Korialstrasz: They will not. Alexstrasza: (to you) (Player), come to me."
 * In World of Warcraft, completing the quests leading up to the Battle of the Wrathgate leads to this exchange. A bug, however, temporarily allowed players to understand the spoilered Draconic part, which alluded to events that were only explicitly revealed in the Fall of the Lich King patch.

Web Comics

 * In a Movie Comics strip based on Windtalkers, one of the two characters pretends to take the other prisoner and deliver him to the Japanese. The Japanese soldiers converse in Japanese and decide to kill them both, unbeknownst to them.
 * In Kevin and Kell, Kell and Frank speak Feline while discussing to prevent their fleas from catching word of this.
 * Attempted by Lucien and Luther in Bite Me..

Western Animation
"Bart: Why don't you let us watch TV, and get her a Roofi CD? Lisa: [in Pig Latin] Bart, don't tell Mom Roofi has CDs! Bart: [in Pig Latin] Why not? What could go wrong? Lisa: [in Pig Latin] She'll buy them, stupid! Marge: [in Pig Latin] You know, I was young once, too. Bart and Lisa: [in Pig Latin] Crap."
 * In The Boondocks fastball episode the visiting Chinese team convince Huey to throw the game or else they'll get sent to death camps. This backfires when they gloat about it in Chinese in his presence, he responds (in Chinese) that he doesn't like to be laughed at. Cue asskicking.
 * Invoked in an episode of Arthur, where each little sub-group invented their own language and only let a few people in on it. Arthur & Buster, Francine & Muffy, etc. Eventually they were so separated that nobody could understand anybody else - except George, who spoke in English because he hadn't been invited into anybody else's private little club.
 * In The Simpsons, Bart and Lisa speak in Pig-latin so Marge could not understand them. Of course, she does.

Real Life

 * In World War II, the Japanese used former students who had studied in the US to listen in to American radio transmissions, so Navajo soldiers were used to speak their own language, completely baffling the Japanese.
 * Has been known to be Truth in Television, when both parents speak a language their child does not. Even if the child does speak the same language, if the child hasn't learned how to spell yet, sometimes parents spell out key words to each other.
 * In both world wars, Welsh regiments of the British Army spoke Welsh in clear on radio transmissions, knowing the likelihood of a Welsh speaker turning up on the German/enemy side was vanishingly small. This idea was copied by the Americans, who used native Americans as signallers to similarly confound Japanese and German intercepts. Unfortunately for Welsh regiments in WW 2, Welsh was thought of by the Nazis as a dawn-language of the Aryan peoples, and a School of Celtic Studies had been set up at one German university. Once the Germans cottoned on, they had a certain number of Welsh speakers available to monitor radio transmissions... the practice was, however, maintained by Welsh units in the Far East. It is no longer used by the British army as so many countries have universities that teach Celtic languages. It did not go un-noticed that a fairly recent enemy, Argentina, actually has a Welsh-speaking minority population. And at least one university in Northern Ireland has a Celtic Studies department offering Welsh language as a module. For that reason, Welsh units serving in Ireland were strictly ordered to observe standard radio operating procedure and not to think of using the old trck, as it is just too well known.
 * IIRC Yevgenia Ginzburg, in her memoir Journey into the Whirlwind, claims she said something to someone in french in front of the NKVD watching over her. She had a feel for her individual guards and said she wouldn't have tried this with just any of them.