Bilingual Dialogue

For whatever reason, two or more characters can't speak the others' language in anything better than heavily mangled, imprecise gobbledygook. However, all involved can understand the others' language when spoken. This results in unnervingly cool, creepy, or cantankerous scenes depending on the set-up, as you have two people listening and responding to each other in completely different languages, creating a Bilingual Dialogue (and sometimes acting as interpreters to any third parties present.)

What you end up with is a conversation that goes something like this:

Annette: "La plume de ma tante est sur le bureau de mon oncle."

Benny: "Oh, I'll go get it, then. Do you want anything else while I'm up?"

Annette: "Non, merci, c'est tout."

This can be Truth in Television. If you learned a foreign language by speaking it (as is common in many schools which use a listen-and-repeat teaching format), rather than by learning to read it, it is easier to passively understand what someone else is saying than to actively generate language oneself. If each person understands the other's first language, it's easier to use this type of conversation than for one person to struggle to speak in a more uncomfortable second language.

See also Bilingual Bonus and Eloquent in My Native Tongue.

Anime and Manga

 * In Magical Girl Lyrical Nanoha, all the humanoid or animal like characters speak Japanese whereas the various talking machines speak either Midchildan or Belkan (represented as bad English or bad German, respectively). There most likely some Translation Convention going on here as Midchildans on Midchilda would presumably speak Midchildan to each other. Although why the machines don't get translated is unexplained.
 * In Tokyo Godfathers, the Hispanic-immigrant mother only speaks Spanish, and Miyuki only speaks Japanese (plus a little English that she learned at school), but they manage to carry on a conversation (with the Spanish unsubtitled for either the Japanese or the English-speaking audience).
 * Subverted in Gigantic Formula. Every single one of the voice actors are Japanese, yet try their best to speak their characters' own languages. Later on in the series, the resident Russian pilot speaks dumbed down Japanese and claims he just learned it because of his being an international ambassador of sorts. By the end, all the pilots speak in Japanese, using headset-things called "automatic language interpreters" to translate their language for them.
 * Played with in Hana Kimi when Nakatsu and Julia got into an argument, Julia speaking English and Nakatsu speaking Japanese. Julia can probably understand most of what Nakatsu is saying but Nakatsu cannot understand a word Julia says, yet they manage a back and forth argument. Mizuki and Sano even think 'Wow, they're fighting on emotion alone'.
 * In Saiyuki Hakkai appears to understand Hakuryu/Jeep's chirps, although none of the rest of the party do nor the viewers. Hakkai occasionally acts as interpreter.
 * In the Pokémon anime, most of the Pokemon themselves speak in languages consisting entirely of variations of their own species name (a Pikachu, for example, can only say the word "Pikachu" or single syllables of that word). While the human characters are consistently shown to be unable to understand this speech in most cases, there are numerous instances where two Pokemon of different species will carry on a conversation where they understand each other perfectly despite each being unable to speak each other's languages.

Film
"Nunb: I'm not getting any reading on the shield. Lando: We've got to be able to get some kind of reading on that shield, up or down. Nunb: I think they're jamming us. Lando: How could they be jamming us if they don't know... if we're coming."
 * Star Wars does this fairly often. Aliens or droids will speak an unintelligible language, and a human character will understand and respond in English. This is usually due to the nonhuman species not possessing sufficient mouth or throat structures to pronounce Galactic Basic. Sometimes the alien speech is subtitled, and other times you just have to infer what's being said by the English responses.
 * Han Solo does this a lot. His conversations with Chewbacca all play this way, which is justified by the fact that they're long-term companions. He also does this with a repair droid on Hoth, and with Greedo, though Greedo's speech is subtitled.
 * R2-D2's chirps and beeps are understood by everyone. In the novels, it's explained that Artoo communicates with despondent beeps, inquisitive beeps, affirmative beeps, etc. and people just get the gist of what he means. In The Empire Strikes Back, however, Luke has a text screen in the cockpit of his X-Wing that gives him a translation of R2's beeps.
 * Jabba the Hutt has similar bilingual conversations with several characters, though he is one of the few aversions to the trope by using C-3PO as a translator. When he's not using a translator, his dialogue is subtitled. At one point, Jabba uses C-3PO in a conversation with an alien who speaks neither Huttese or Basic, but understands the latter. In this case, it is a trilingual conversation, since both Jabba and the alien can understand Common, meaning that not only can C-3PO convey what each is saying to the other, they can also make sure that they are being accurately translated..
 * Jabba's Twi'lek stooge Bib Fortuna, whose dialogue with Luke in Return of the Jedi is not subtitled. Since Luke has Fortuna thoroughly mind tricked, it's not hard to work out what he's saying.
 * Lando and Nien Nunb, his Sullustan copilot in the Death Star run in Return of the Jedi. Nien Nunb is actually speaking Haya, a real African language, giving a Bilingual Bonus on their full conversation, which is full of Repeating So the Audience Can Hear:

"Heyward: You there! Scout. Must. Stop. Soon. Women. Are. Tired. Magua: Three leagues. Better water. We stop there.
 * Gordon from Godzilla: Final Wars speaks English throughout, as does Kazama to a lesser extent. No one, not even an alien, ever has difficulty understanding them, nor them anyone else. It's averted in the English dub, because everyone is speaking English.
 * Tiny bit in Die Hard. The French terrorist says something in French over the phone to the American woman. She replies in English.
 * In Ichi the Killer, characters speak in Japanese, Chinese and English (at least) and all seem able to understand each other just fine. Which is just as well, since there are plenty of other things to misunderstand.
 * This occurs in all three of the Ocean's movies with Yen. In Ocean's Eleven, Danny Ocean (George Clooney) and Rusty (Brad Pitt) can understand Yen's Chinese, but always speak to him in English, which he understands. This extends to the entire crew in the sequels.
 * Subverted in Ghost Dog: The Way of the Samurai. The title character speaks English to his best friend, who speaks French back, but they don't understand a word of each other's language. Even still, they seem to share a psychic connection and are always talking about the same thing.
 * Played With in Last of the Mohicans, given the Indian penchant for Dry Humor. Native characters slide in and out of multiple languages, including Huron and Mohican, which are subtitled. Magua, being a Huron enslaved by Mohawk who became a French war captain turned British scout, speaks more languages than anyone else; in the final parlay, he alone understands the whole conversation (he is not happy). It's both subtitled and translated by Heyward, who speaks French -- but not Huron -- and he deliberately mistranslates a key piece of dialogue. Then there's this exchange, which doubles as a Big Entrance for Magua:

Heyward: No, we stop in the glade ahead. Understand?

Magua: (subtitle) Magua understands the white man is a dog to his women. When they grow tired, he puts down his hatchet to feed their laziness.

Heyward: What did you say?

Magua: Magua say, "I understand English... very well.""


 * In 1941, the Japanese crew of a submarine speak only Japanese, and a German on board speaks only German. Somehow, they understand each other perfectly. It's a pretty goofy movie.
 * Also in Sayonara Jupiter. All the characters there have Translator Microbes clipped to their lapels.
 * In{Seachd The Inaccessible Pinnacle a friendship is struck between an exiled Scotsman and a shipwrecked Spaniard, despite the fact neither understands the others language.
 * Happens in some Canadian movies, most notably Bon Cop, Bad Cop, a send-up of buddy-cop movies, linguistic tension, and hockey fanaticism, and The Rocket/Maurice Richard, the biopic of the legendary French-Canadian hockey player. Sometimes it's played for laughs or dramatic tension, but for the most part, you simply have different subtitles on the bottom of the screen, depending on the version of the film, and the dialogue stands on its own.
 * Several of Wong Kar-Wai's movies feature bilingual dialogue, the most notorious being 2046. This movie casts several Hong Kong actors, Chinese actors and one Japanese actor, all of which speak their own native language throughout the movie. The whole movie is filled with (seemingly) perfectly natural conversations where one person speaks Cantonese and the other speaks Mandarin. Not to mention that while most of the movie is narrated in Cantonese, the first five minutes is narrated in Japanese, making the movie virtually impossible to watch without subtitles.
 * The Pai Mei training scene in Kill Bill. She even says flat out she only speaks 'a little' Cantonese, but he continues to speak as if she's fluent... and she understands every word he says, apparently, without any problems. In return, she speaks with extremely halting and limited Cantonese and Mandarin peppered with English and he has no problems understanding her.
 * In District 9, the humans and "prawns" clearly lack the vocal setup to speak each others' language, but the Prawns understand English perfectly well and human workers in the District understand the alien language.
 * In Michael Moore's Sicko, Moore does this with the French, who can understand his English.
 * Kukushka is based almost entirely on bilingual (sometimes trilingual) dialogue. The three protagonists only speak and understand one of Russian, Finnish and Sami, so neither understands what the other two are saying. This makes for some interesting misunderstandings, some with tragic consequences.
 * In Ridley Scott's Robin Hood, Isabella of Angoulême speaks mostly in French, and everyone else responds mostly in English.
 * Used Extensively in Man on Fire. Justified, since Creasy can speak Spanish and it's a border town, so most people will understand English anyway.
 * In the movie The Fifth Element, Father Cornelius is able to understand Leeloo's rapid-fire delivery of her, "divine language." And she has no problem understanding his responses in English. Leeloo does eventually learn how to speak English, to a point any way.
 * In the movie Larger Than Life, Jack meets A Native American Chief. Chief says something, which is subtitled to English, then Jack replies, which is subtitled to Navajo.
 * It's pretty common in the movie Machete, but specifically for Those Two Guys who work as dishwashers. One will of the guys understands what his friend says in Spanish but will only answer in English.
 * Played with in The Beat That My Heart Skipped: The French-speaking protagonist and his Vietnamese-speaking piano teacher seem to understand the gist of each other's words, even though neither speaks the other's language. See them have a bilingual argument here.
 * In Seducing Mr. Perfect, Robin, the American protagonist, speaks English almost exclusively but can understand several different languages. Meanwhile, Min Joon (sp.?), the South Korean protagonist, has some difficulties speaking English but can understand Robin as long as he doesn't use American euphemisms and slang. Her response to him accusing her of "doing it" with someone was to genuinely wonder what he was accusing her of doing with said person.
 * Bon Cop Bad Cop, being a movie about the relationship between Ontario and Quebec as much as it is about the serial killer, has two bilingual protagonists, and several Francophone and Anglophone supporting characters. The dialogue of the movie is nearly equal in French and English, and there are a whopping SIX subtitle options on the DVD, for Francophones, Anglophones, and Bilinguals, with hard of hearing options for all of the above.

Literature
"Sir Houston Carbarn smiled. "(-1)(-1) = +1," he informed. The Starboard Admiral slammed his palm against the desk. "Of course! The principle of the double negative! Two negasphere's make a posisphere! Our Gray Lensman has genius, Sir Houston!" "?" agreed Sir Houston."
 * Wedge Antilles runs up against his inability to understand Wookiee in one book of the X Wing Series. Fortunately there was a translator droid on hand, even if he was rather more abrasive and sarcastic than C-3PO.
 * In Richard Feynman's biography "Surely You're Joking, Mr. Feynman!", he says that when he taught Physics in a Brazilian university, it was easier to communicate with the students when they tried their usually crude English with him and he used his definitely crude Portuguese with them. He notes that it's easier to understand your own language said in an awkward way than hearing another language in its original form, even knowing that language well enough to speak. (That could be a more realistic way for Hollywood to portray it).
 * One of the recurring Discworld characters is the Librarian - a wizard turned into an orangutan early in the series. He understands Morporkian (English) perfectly well, but always speaks in orangutan, using words like "Ook" and "Eek". Most human characters have little trouble understanding this, but every now and again someone unfamiliar with the Librarian meets him and cannot quite figure out what that particular "Ook" meant. The Librarian tried to address this issue by writing an Orangutan-Morpokian dictionary, but has not progressed beyond "Ook" yet.
 * There's some serious Fridge Logic in one of the Dinotopia novels by Alan Dean Foster. We're told that dinosaurs speak a number of different languages depending on the species, and most can't manage a human tongue. Okay. Similarly, humans can't usually speak the dinosaur languages. Fine. But there is a wandering tribe where the humans and the dinosaurs work together closely to survive and have for years - and neither species understands what the other says at all. Argh!
 * Translation Convention means everyone "speaks" English in A Tale of Two Cities, but an in-story version of this trope happens when Madame Defarge barges into Lucie's house, intending to have her Revenge by Proxy, but is stopped by Miss Pross. The narrator tells us that both women spoke in their native language, but their facial expressions and body language makes their intentions perfectly clear.
 * Played for laughs in Backstage Lensman by Randall Garrett, a take-off of Lensman Space Opera. Sir Houston Carbarn is the most brilliant mathematical physicist in the known universe; one of only a handful of living entities who can actually think in the language of pure mathematics.


 * Used in Jane Eyre. Adele often speaks in (untranslated) French, to which Jane responds in English.

Live Action TV

 * Michael and Jin from Lost communicate in this manner while building a raft. Sawyer, predictably, calls them Han and Chewie.
 * On one episode of The Muppet Show, in which a bunch of Funny Foreigner muppets were guests, the Swedish Chef was able to converse with them with ease, even though no one else could understand them. Simmilarly, the Chef has been shown to understand/translate for the even more unintelligible Beaker.
 * In Father Ted, the 'monkey priest' can only communicate in 'ooos' and 'aaaas' but all the priests understand him perfectly.
 * Sort-of employed in Firefly, where English and Chinese are claimed to be spoken in equal measure by everyone, but in practise the latter finds more use in unusual euphemisms.
 * Portrayed quite realistically in Deadwood, as Al Swearengen and Mr. Wu (whose only understanding of English is mostly limited to the word "cocksucker") are able to communicate reasonably well through pantomime and drawings, as well as their clear emotions.
 * In the Korean Drama Bad Boy, Moon Jae In is in Japan, talking with the artist in japanese, with both Korean and English subtitles underneath.
 * In an early Magnum, P.I. episode, Magnum had a client who was a Chinese woman. She spoke Cantonese, while her 'father' spoke Mandarin. This happens a lot in television shows, because they assume the audience won't be able to tell the difference.
 * Have we all forgotten Cousin Itt? (Could also be put under movies).
 * In the Korean drama, Dae Mul, the Korean president is able to communicate with the American president and vice versa without the use of translaters. Same goes for her and the Chinese leader whom she tries to negotiate sparing the lives of the Korean submarine who invaded his country.
 * In another Korean drama, Sweet Spy, the male lead is a Korean-American actor. He spends the whole drama chatting away in English to his assistant, who talks back to him in Korean.
 * Averted in 'Allo 'Allo! where the entire show is in English, but the characters all speak their own language (French, German, English, Italian) which the people who don't speak that language can't understand. (with the exception that the French and Germans can understand each other. - it could be that it is assumed all Germans know French and all French know German, especially during the occupation.)
 * Betty White and an African tribesman switch between an African language and English mid conversation in The Tag at the end of an episode of Community. This is so another tribesman won't be spoiled by the English half of their discussion of Inception.
 * In an episode of Boardwalk Empire, Lucky Luciano mutters a comment in Yiddish to Meyer Lansky, who responds in Italian, showing their close relationship in spite of their different ethnic backgrounds.
 * In British young kids' show Tots TV, one character spoke entirely in French while the others spoke English.
 * A staple of The River, which makes sense as it takes place in South America.
 * In Danger 5, one of the eponymous team of five is Ilsa, who speaks to everyone in Russian. Her English speaking companions always speak to her in English (even the European Pierre). Hitler and various goons speak German, and everyone understands each other perfectly. All in all, the series contains correct English, German, Russian, Italian, French, Japanese, and Cantonese, a result of being from Australia's premier multilingual channel, SBS.
 * Sooty, A British children's puppet show, has a dog called Sweep who speaks only in squeaks, but everyone else converses with him in English perfectly. The audience is given a translation by the fact that whenever he talks to someone, they repeat what he says anyway, but long-time viewers can translate him quite well via context and how many syllables he squeaks, since he actually does have a script for his words.

Music
"Cop: Where are you going? Max: Sto portando al mare le ragazze. (I'm taking the girls to the beach.) Cop: What girls? Driver's license and registration, please. Max: Va bene. Subito. Un secondo... (All right. Immediately. Just a second...)"
 * Played straight in the video of "La lunga estate caldissima" by Max Pezzali, set in Los Angeles, where, in the end scene, a cop stops Max Pezzali and the following dialogue happens:

Professional Wrestling

 * Independent wrestler Delirious speaks in tongues that are completely incomprehensible to the audience, but referees, managers and fellow wrestlers have and do understand him and hold conversations with him (occasionally during a match).

Video Games

 * In Escape from Monkey Island there's a drunk who speaks in unintelligible gibberish that Guybrush apparently understands, you can infer what's being said based on the dialogue options you're given.
 * In Knights of the Old Republic, the player character is surprised when Mission speaks Basic instead of Twi'lek, and is told that a lot of aliens can do it, but most don't. A very minor human Sith character on Korriban speaks Twi'lek and tells the player that it's because his Basic is so bad and everyone understands Twi'lek anyway. All alien languages that the player understands have subtitles.
 * Dr. Breen's contact with the Combine Advisors in Half Life 2. The Advisors' language sounds like unintelligible growling and slurring - and we get no translation of any kind. In fact, it's implied it has a psychic component to it which might explain how Breen understands it during his dialogue with one even though the Advisor doesn't speak in that scene but speaks in others.
 * In Ultima Underworld, all three varieties of lizardmen can understand English, but only one variety can speak it. The player can earn the lizardmen's favour by attempting to speak their language.
 * The various characters in the Tekken games speak English, Japanese, Korean, or Mandarin in their cutscenes and never seem to have the slightest trouble understanding each other (though subtitles are provided for characters who are not speaking the player's native language). Even novelty characters who communicate with clicks or animal noises manage to get their point across just fine.
 * Virtua Fighter has done pretty much the same in the later installments.
 * Happens with the English-speaking characters in Gun Hed.
 * And Gun Hed itself: 'Gun Hed Pilot: [speaks subtitled Japanese]; Gun Hed: 'You could say that, yes.'
 * In Ico, Yorda and Ico speak completely different languages, neither of them English nor Japanese; Ico's speech is subtitled in the player's language, and Yorda's in strange hieroglyphics. Luckily, "Don't worry, I'll catch you", "I'm tired, let's rest here", and "Holy crap, shadow monsters are trying to drag me off to some unspeakable netherworld!" are fairly easy concepts to get across.
 * In Fire Emblem Path of Radiance and Radiant Dawn, Leanne speaks the ancient tongue and has a great amount of trouble with modern tongue. The few times she speaks the modern tongue, she shows her difficulty ("I...fight...yes?"). However, even the handful of other people who are fluent in ancient tongue usually adress her in modern tongue, and she has no trouble understanding exactly what they're saying in a language that she is physically able to speak, but apparently is very bad at.
 * In Mega Man Star Force 3, Solo's new EM-being Laplace only speaks through buzzing noises. Solo can understand him perfectly, and even tells him to shut up at one point.
 * A conversation late in Yakuza 3 plays out like this using English and Japanese.
 * Heard between Jorge-052 and the Hungarian civilian Sara Sorvad at the end of the first mission in Halo: Reach.
 * The cutscene in Saints Row 2 introducing the leader and The Dragon of the Ronin gang consists of the former (Shogo) speaking English while the latter (Jyunichi) speaks unsubtitled Japanese, though this stops after awhile when Shogo tells him "we're in America, speak English"; from that point on, while Jyunichi does speak in both English and Japanese, which one depends on which language the other person in the conversation is using.
 * In The Legend of Zelda the Wind Waker, Jabun speaks ancient Hylian while the King of Red Lions speaks modern Hylian (which is of course rendered as whatever language you're playing the game in), making only one side of their conversation understandable and keeping everything incredibly vague. In the New Game+, all ancient Hylian is translated, including Jabun's.
 * Averted in World of Warcraft: all Horde characters speak a native language and Orc (except orcs, who only know the latter), and all Alliance characters speak their native language and Common (except humans, who only speak the latter). The default languages are Orc and Common, but as a player you can switch to the other language your character knows at any time; anyone who also knows that language will then see exactly what you type into the chat just like you were using the default language, but players whose characters don't speak your language will see semi-random gibberish instead.
 * The most famous of the gibberish is probably "kek", which is what Alliance players see when a character speaking Orc says "lol".
 * Of course, non-player characters appear to speak all languages—though that could be explained by some having learned both Orc and Common, something player characters cannot do.
 * A rather bizarre example in the Fallout: New Vegas DLC "Old World Blues. The Courier, if he/she has a high enough intelligence, can be able to converse with Dr. 8, a brain in a jar that can only communicate using Rob Co code script. The player won't understand a single word he says regardless, but the Courier's responses give enough of an estimate.

Web Original
"Lopez: (Spanish) ...Hello. Sarge: Simmons, where in Sam Hill have you been? Lopez: (Spanish) Cave...study.
 * Pom Pom and The Cheat, Homestar Runner and Strong Bad's friends respectively, are both unintelligible, but everyone else can understand them fairly well (and the other way around). However, when The Cheat loses a bet to Strong Bad, he says something that sounds a lot like "Yeah, yeah!", and in colouring, he says something that is unmistakably "Okay".
 * And in the Powered By The Cheat (cartoons made by The Cheat), the voices are in English, despite it being The Cheat doing them.
 * Discussed in Cracked, which calls Bilingual Dialogue The 5th Stupidest Way Movies Deal with Foreign Languages.
 * Played with in Red vs. Blue. The Reds can never understand Lopez, until he poses as Simmons.

Grif: You sound weird.

Sarge: Yeah, almost like he's speaking a foreign language. But he's speaking very slowly and clearly, so I understand what he means.

Grif: Me too."

Web Comics

 * A interesting take on this is given in the webcomic Experimental Comic Kotone from Tsunami Channel, with the Japanese traditional girl Haruna and Onii-chan. In a point of the story, each began to understand what the other is saying, despite not having a grasp of the other one's language, and both talking in their respective mother language. But when they had a serious misunderstanding for the very first time, their understanding suddenly and dramatically stopped. The issue was solved later, but since then Haruna has began to learn English.
 * In the webcomic Starslip Crisis, the good gentlemonoliths of the Council respond to all conversation in telepathy that the reader is not privy to, making for some good Newhart-style one-sided conversations.
 * Subverted in Doug TenNapel's Gear: Waffle the cat and Chee the mantis seem to converse in spite of their language barrier, and Waffle even has a change of heart immediately after one of Chee's comments. Then, as he turns to leave, Waffle adds, "By the way, I haven't understood a thing you've said."
 * Bob and George's Future Alternate Bass has this with an Expy of R2-D2, Mettool D2.
 * Bob has it too
 * In Tales Of Gnosis College the Sultan of Pazar speaks in English to a Russian girl who replies in her native language.

Western Animation
"Lady Rainicorn: "Remember when we ran naked through that field? That farmer was so offended!""
 * In the obscure French cartoon Fly Tales, all the characters speak gibberish, which sometimes can sound very close to French.
 * While Más y Menos from Teen Titans speak only Spanish, only about half the cast can understand them. Control Freak even got so frustrated he changed their language to English using his remote!
 * The Incredibles: "Fly home, Buddy. I work alone." "Et ton costume est complètement ridicule!" Although it wasn't really a conversation, as the only person who understood French was the one speaking it.
 * Done in Angry Beavers by masked wrestler El Grapadura. (El Grapadura can be loosely translated to mean 'Stapler'.) In addition, Grapadura stars in the episode 'Norberto Y Dagetto en El Grapadura y el Castor Malo', which is entirely in Spanish and based off of El Rey detective movies.
 * Grapadura is again seen in the episode Pass it On, where he speaks Spanish to Dag. His subtitles are in Korean, and Dag calls him Swedish. He also throws out a bit of Gratuitous English. ('Ey, baby!')
 * Lady Rainicorn of Adventure Time speaks Korean. She is never translated on the show, and most of the time, characters will give a good idea of what she is saying. Jake appears to understand her perfectly well and even speaks Korean back to her. There is also a case of Bilingual Bonus and Getting Crap Past the Radar, as Finn asks her to tell a joke, and she complies with a rather embarrassing anecdote that Jake says "does not translate well".

"Monogram: "How much is this costing us, Carl?" Carl: "Une petite fortune." Major Monogram: "Petite." That means small, right? Well done, Carl.""
 * Played with in Phineas and Ferb. Carl hires a bunch of French maids to clean the OWCA. Carl can apparently speak and understand French, but Moogram does not, so when questioning Carl on the cost for the maids he misunderstands him... but is satisfied none the less.

Real Life

 * Common for speakers of dead languages with Latin as one notable example - one can be quite competent in translating from Latin, but actually generating original Latin can be quite difficult, since students are rarely drilled in English-to-Latin translations and there is virtually no demand for such skill in real life. Often in Latin class the teacher/textbook will ask questions in Latin and the students will speak/write their responses in English.
 * In Real Life, when two people speak one another's language incompletely, each speaking in one language is actually a fairly efficient mode of communication. Each can either speak in their own language, relying on the other to understand correctly, or try to speak the others language, knowing they will be understood correctly.
 * Is a common phenomenon all over South Asia, especially in India and Pakistan, where there are literally hundreds of languages. A good percentage of the population,, tends to be receptively multilingual, meaning they can understand other languages (having had enough exposure to them via Popcultural Osmosis) but may not be able to actively speak in them. So for example if persons A and B meet, and person A can receptively understand person B's language but not actually speak it, and vice versa, they might both proceed to speak in their own languages and comfortably understand each other. As English is the official language of both countries while Urdu in Pakistan and Hindi in India is the "National" language, you have a situation where a person is actually "tri-lingual", they know English, Urdu/Hindi and their regional language and finally they also may speak or have an understanding of many other regional languages.
 * When Sasha Cohen's (not to be confused with Sacha Baron Cohen) coach was Tatiana Tarasova, Tatiana spoke Russian to her, and Sasha (who understood Russian due to her mother being from the Ukraine) responded in English.
 * This was standard procedure for the Apollo-Soyuz missions: the American astronauts spoke Russian and the Russian ones spoke English.
 * Laura Ingalls (creator of Little House on the Prairie, which was Based on a True Story) knew a woman who spoke only Swedish. This woman spoke Swedish to Laura, Laura spoke English to her, and each understood what the other was saying.
 * This is standard practice in classical orchestras: because very often the players are from different countries, communication tends to be a problem. So classical musicians in an orchestra will generally be very multilingual, and, as a courtesy, speak in the language of the person they're talking to. Hence a French violinist talking with the German flautist next to him will speak in German, and the flautist will speak in French.
 * This troper (who is Dutch, though not a classical musician) once had a conversation with a German friend, which soon evolved into said troper speaking German and said friend speaking Dutch. In England.
 * Conversations in which English-speakers speak English and French-speakers speak French are relatively common in Quebec. Actually, even more common are conversations in which English-speakers speak French and French-speakers speak English.
 * When the Hot Line is being used, the Russians write in Russian and the Americans in English, with the other side translating.
 * Spanish, Portuguese, and Italian are close enough that speakers of one of these languages can understand a great deal of what is said in the other two-although Portuguese is generally more challenging to understand due to its shifted phonology. Speakers of Portuguese and Italian are also generally able to understand Spanish better than Spanish speakers understand Portuguese or Italian.
 * Portuguese (European one specially) is, however, the easiest to "mutate" into the others. When spoken with the right accent and basic knowledge of vocabulary and grammar, Portuguese speakers can bake a Portu-Spanish or Portalian which can go through whole conversations.
 * Same goes with Swedish and Norwegian.
 * This was common in the former Czechoslovakia, where the Czechs would speak Czech and the Slovaks would speak Slovak, and each would understand the other. This happens less today, after the country's split, but the two are still very similar, and share close resemblance to other members of the West Slavic group of languages. (Polish, Czech Slovak. etc)
 * Happens a lot between Swedes, Danes and Norwegians, since those three languages are so similar.
 * Same goes for Russian, Byelorussian, and Ukrainian. However, speaking in a sort of pidgin, be it Russian-Ukrainian (called surzhik, which means "flour made of two different types of grain) or Russian-Byelorussian (called trasyanka, with exactly the same meaning) is even more widespread.
 * Literally every Irish speaker in Ireland is bilingual, however, some are more comfortable speaking Irish rather than English, so this tends to happen occasionally.
 * Also found in Northern Wales and in the Outer Hebrides of Scotland, were significant Welsh and Scottish Gaelic-speaking populations, respectively, are found.
 * Also found in the Scottish Lowlands, although how frequently depends on where the line is drawn between the Scots language proper and Scottish English, given that most colloquial dialects are a hybrid of the two, and the strength of the Scots portion varies from speaker to speaker.
 * In schools with a language option, conversations can occasionally be heard between, for example, French and German students in their chosen languages. There's no guarantee that each participant understands what the other is saying though...
 * This is even done when the languages are more different, such as Japanese and English, and sometimes, even three or more total languages. Even native speakers will switch languages to either invite someone into the conversation or block someone out, or sometimes just because a certain statement is easier in a different language. However this does tend to lead to a bit of pidgin language.
 * Happens often in multi-lingual countries. Especially Singapore with its home brew Singlish.
 * It's common for the children of immigrants in any country to understand but not speak their parents' language and vice versa, especially for Filipino-American families.
 * Anyone who's been to or lives in the American Southwest is all too aware of this, what with being right next to the Mexican border. It's not at all uncommon to hear or have entire conversations with English and Spanish being switched at a moment's notice, and without skipping a beat.
 * It's also very common in the more bilingual areas of Canada to hear people switching between French and English mid-conversation, sometimes mid-sentence.
 * Linguists call this "code switching," and it can occur even in mid-sentence.
 * Using a command prompt frequently falls into this. The human says things that look like someone spilled soda on the keyboard, and the computer responds in English. Granted, that doesn't mean you'll understand the output, but "dir \ /p" and "Volume in drive U is AFS" are definitely not the same language. Programming is often even more like this.
 * The French region of Alsace has gone back and forth between France and Germany several times, due to the many wars between the two countries. As a result, most people in the western half speak French and understand German, while people in the eastern half speak German and understand French. Thus creating this trope when talking to each other.
 * If you are bilingual, and if the one you are talking to is bilingual too, you may use the two languages at the same time, choosing the word that fits best, using the grammar of both languages, or literally translating sayings. If someone else who knows only one of those languages listens to you two talking, they might find it VERY awkward.
 * This happens in Korean television programs a lot, where a Korean interviewer will ask a foreign guest a question in Korean, and the guest will respond in his native language with subtitles, giving the impression that each understands the other's language, when all they did was edit out the translations. As time goes on, though, this is becoming less common as Korean interviewers use more English when interviewing guests who do not speak Korean.
 * People who travel abroad with the intention of Second Language Immersion have found that the inverse can happen as well. Let's say, for example, an English-speaker goes to Paris to brush up on his French. The English-speaker will try to speak in French, and the French-speaker, either to be polite or because they want to brush up on their English as well, will continue to speak in English. This can lead to an interesting game of chicken where both people are speaking in their second language to see who cracks first.
 * This happens quite often when school subjects are taught in a different language than most students and teachers use outside of the school. Students switch to their local dialect or first language during a lesson, because certain ideas are easier to explain that way or they don’t know a word, and the teachers keep asking question in the school's chosen language.