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.
Subtrope of Language Barrier.
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"):
Foreign Speaker #1: Xmphlaca bi flucalaka un bijnana y aquaholder.note His head is like a bucket Foreign Speaker #2: Gne!
He also mentions that insulting them in a language they can understand is even ruder.
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.
"I'm from a different world, y'all," I said flatly. "If you don't like it, that sucks for you. I'm also the only person who knows right now that everything's going wrong, and I'm the only person who can fix it. So you better start liking me real quick before I decide I've had enough and go off on my own."
Marks of Time, chapter 24, has a scene where Lyla speaks Russian with a woman from Moscow. They pretend to talk about Fort Collins. The real topic is a man with them, who understands English but not Russian.
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.
Done again (by the same actor, Christoph Waltz) in Django Unchained, when Schultz, a native speaker of German, takes advantage of the fact that Damsel in Distress Broomhilda speaks German so that he can tell her of the plan to rescue her without worrying whether anyone else is listening in.
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.
A scene in My Fellow Americans does this. James Garner's character (an ex-President), and Jack Lemmon's character (also an ex-President) are on the run from assassins and hiding in a truck full of illegal immigrants (It Makes Sense in Context, sorta). While there Garner strikes up a conversation with one of the immigrants in Spanish, finally making some insulting remarks about Lemmon. As the punchline, after the two part company with the immigrants, Lemmon respons in flawless Spanish, "I can speak Spanish too, dickhead!"
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.
In one of the Ender's Shadow sequels, Bean and Petra are taking a cab in Armenia and start speaking in Portuguese (they had previously spent some years in Brazil). Bean points out that they're being very rude to the cab driver.
In the Warcraft Expanded Universe novel, Lord of the Clans, another orc tries to tell Thrall "Run! I will protect you!" in Orcish, but Thrall, who doesn't know Orcish, does not understand what he says until someone tells him later, and neither does the reader.
This appears in Jules Vernes' 20,000 Leagues under the Sea, as the crew of the Nautilus communicate among themselves in a secret language they invented to keep outsiders out. One of the crewmen breaking down and asking for help in his native language, revealing himself to be a Frenchman like Professor Aronnax himself serves as a significant plot point.
Happens in The Bible, when the Assyrians are besieging Jerusalem. The Assyrian officers who come with surrender terms are asked by the Jewish commanders to converse in Aramaic, since most of the common folk don't understand it. However, at least one Assyrian knows Hebrew and shouts his "surrender or die" boast - listing what fates have befallen Assyria's previous enemies - loudly and in Hebrew so that all the defending soldiers will hear it too.
In Sergey Lukyanenko's Seekers of the Sky duology, two characters try to converse in Gallic (French) while being given a tour of Aquincum (Budapest) by a local young man. Halfway through the conversation, he asks them in flawless Gallic if they'd like him to step away, so that they may continue their conversation in private. It turns out that the kid is an Omniglot, speaking eight languages while teaching himself a ninth.
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.
One episode has a Chinese mother and daughter talk in Mandarin so House won't understand. Of course, he speaks it, making it a case of Bilingual Backfire.
House blackmails 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.
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.
When Barney first met Ted, he has Ted pretend to be his deaf brother to impress a girl. She starts signing at Ted (who signs back). Barney believes that Ted is playing him up to the girl, when he's really telling her that Barney's a liar, having her give him a fake number.
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.
Subverted again in another episode. Elliot wants to speak with Turk in private about allowing a small boy to watch the upcoming operation of his father, but can't discuss it openly as said boy is in the room with them. She tries to speak to Turk in French to get around this, but Turk only learned enough French in high school to make cheesy (and grammatically incorrect) pick-up lines, so the attempt fails.
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.
In "The Even Chance", a French captain of a captured ship says to Hornblower that he swears he won't try to gain the command back again, and says his men won't interfere either. He in fact instructs them to wait until his signals them to take over the boat. Which doesn't work out great for them as Hornblower understands French himself and prepared for the situation. Reality Has No Subtitles and viewers who don't understand French don't know what was said.
Hornblower and the Duchess openly discuss the Admiralty's super important dispatches which she's hiding. They are standing very close to a Spanish guard who's in hearing distance. Common soldiers probably didn't speak English, but Don Massaredo did and many a conversation was conducted in English. Luckily, he apparently didn't understand.
Lucky Dog 1 has many of the bilingual Italian-speaking characters switch to their native language when they're around monolingual English speakers and don't want to be overheard talking about the Mafia or gang-related affairs. However, one of the few instances where they seemingly forget to stop speaking English while discussing these confidential matters with one another inadvertently leads to an old woman overhearing, and realizing they're convicts on the run. This can lead to them being forced to silence her permanently.
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.
Also with upper and middle class employers of lower-class servants. Thus in German the French phrase "pas devant les enfants" or "les domestiques" was semi-proverbial until ca. World War 2, when French was still the foreign language most likely to have been learned in the "educated classes".
Also Truth in Television when someone who is bilingual encounters someone they don't really wish to have a conversation with/they want to be left alone or the like. Some Japanese rock musicians do this in English-speaking locales since people expect them to not know English - giving them an easy out for encounters with non-Japanese-fluent fans or media. Pata of X Japan is famous for doing this - he has good enough English skills to work with English-speaking American artists and to pick up English-speaking groupies, but is also a Shrinking Violet and will happily pretend to be unable to speak English to avoid an encounter he doesn't want.
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. (On one occasion when the 3rd Battalion Welsh Guards was fighting in the battle of Monte Cassino its positions were showered with leaflets inUrdu soon after its radio messages had been intercepted by Germans.) 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 trick, as it is just too well known.
Similar to the above Welsh example was Irish during the early years of the Congo Crisis. Irish peacekeepers deployed in the 1960s would radio secure communications in Irish so as to provide an extra layer of security towards whoever was monitoring, whether Belgian, American or Congalese.
Sports teams can employ this trope to keep their tactic a secret from the opposition. For example, South Africa has a whopping eleven official languages and English is generally used as the lingua franca. But when their sports teams play against English-speaking countries such as England, Australia or New Zealand, they sometimes use Afrikaans.
In World War II, the Germans began looting art from German-occupied France. Rose Valland, a French woman who was the overseer of the Jeu de Paume Museum, where the looted art was being stored, faked being unable to understand German. For four years, she secretly recorded where 20,000 French and Jewish art were being sent.
A cant, or cryptolect, is the jargon and slang of a group used to hide communication from people outside that group. Basically they create their own language barrier to hide behind. Various criminal subcultures around the world have used a thieves' cant to speak to each other without fear of being understood by victims or the authorities.
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.
Korialstrasz:My queen, do they know?
Alexstreasza: No, my beloved. (Draconic) They must not discover the fate of the young paladin. Not yet.
In Stand Still Stay Silent, five languages are spoken in the story world, so this happens often. At one point, Tuuri tells a train conductor in Swedish to pick up her cousin (who only speaks Finnish) and carry him into the train.