Suppose Alice knows a language (either real or invented) Bob does not. Therefore if she is in a playful mood (and/or has feelings for Bob), she may deliberately speak it to him (or in front of him), knowing that it will puzzle him. May sometimes be used for comic effect: for instance, Alice may imply that shes saying something offensive about Bob, while in reality it would be exactly the opposite (like I love you).
Unlike Hiding Behind the Language Barrier, when the aim is to transmit information from person A to person B without person C eavesdropping, in this case the whole purpose is to make person C intrigued and/or frustrated (person B may not be present at all). And while the former trope may be invoked in very serious circumstances (like communicating in front of an enemy), this one is usually applied for good-natured teasing between friends or family members. It is also quite often employed in romantic relationships, especially by the female side. Because it is common assumption that every woman should be a mystery for her man, and theres just something about a beautiful girl saying cryptic things in an exotic language you will probably never understand.
Subtrope of Cryptic Conversation and Language Barrier. Compare Delighting in Riddles, when a character speaks in understandable language, but in a very cryptic way, for exactly the same purpose. Also compare Loves Secrecy, when a character withholds information for the sake of puzzling others. If the subject actually knows the language the teaser is speaking, this becomes Bilingual Backfire.
In Real Life, code languages, like Pig Latin, often served this purpose—both on elders' and on children's part. Another notable example is shorthand: many wives and mothers employed it to write their Christmas lists so that it wouldn't be necessary to hide them from husband and children, and some of them deliberately left them in plain sight to make everyone wonder.
- Kaguya does this in the extended version of chapter 20 in Kaguya-sama: Love Is War, where she admits something to Shirogane. Since it's shown from Shirogane's point of view, the dialogue is rendered as random shapes for the audience. He never finds out what it means, even when he tries to look it up in a French to Japanese dictionary.
Kaguya: (thinking) That wasn't French.
- Neon Genesis Evangelion R: During one of Asuka's rants, Shinji asks why she's so angry with him, and she replies "Because... ich liebe dich, you jerk!" ("I love you" in German). A while later Shinji tells her that he looked up what the phrase means and therefore has known for a while how she feels about him.
- Psalm of the Lark: After a guy tries to flirt with Harley using the line "Mi casa es su casa", Harley disses him in Spanish by calling him stupid. Luckily, he can't actually speak Spanish.
- Control: In high school, Yukari would tease others in foreign languages, usually English. Most of the people only knew Japanese.
- Two Twili attempt to mock Zelda and Link behind Impa's back in Their Bond, but it backfires when she asks them to repeat it in Hylian. She is not happy when she learns what they said.
- Once: when the Guy asks the Girl whether she loves him or her husband, she answers in Czech: "Miluju tebe" ("It is you I love"), and archly refuses to translate it.
- In George of the Jungle, the three African guides frequently insult and mock Lyle in Swahili, knowing he can't understand a word they're saying. At first it's believed they can't speak English, but later on it's revealed they are all fluent, and were hiding it just to annoy him. (It also reveals that some of their earlier insults, which seemed to coincidentally respond to something Lyle said, were in fact deliberate.)
- In Astrid Lindgren's Bill Bergson (Kalle Blomquist) series, the three "White Rose" protagonists employ the Swedish language game Rövarspråket to mock three other kids, belonging to "the Red Rose". Hiding Behind the Language Barrier is also invoked later, when they have to communicate in front of a dangerous criminal.
- In James Clavell's Tai Pan, the mandarins—the Chinese officials—"renamed" the highest British official His Excellency Longstaff: they translated his last name into Cantonese as "Odious Penis". This nickname was used in all official letters addressed to Longstaff for more than a year.
- In Magicae Mathematica series by Jim West, the protagonist Alex was taught Latin by her mother; they often spoke it in front of their husband/father just to tease him.
- In an episode of Babylon 5, Marcus Cole told Susan Ivanova "You are the most beautiful woman I've ever met" in Minbari, saying that it was a Minbari greeting. In a later episode, it's revealed that Ivanova has an eidetic memory, and in that time she's actually learned a bit more Minbari, enough to know what it really means.
- In the 1980's Mike Hammer tv series, a woman from a Fictional Country flirts with Mike in her own language, then implies that what she just said is rather naughty. Mike repeats the phrase to another beautiful babe who's quite shocked and asks if he knows what that means. When Mike admits he doesn't, she offers to demonstrate.
- Blanche from A Streetcar Named Desire invokes this at one point: she asks Mitch whether he speaks French, and when he gives a negative answer, she says: "Voulez-vous coucher avec moi ce soir? Vous ne comprenez pas? Ah, quel dommage!" ("Would you like to sleep with me this evening? You dont understand? What a tragedy!")
- In one Inside the Actors Studio issue, Natalie Portman said a line in Hebrew (at the very end of this interview part) and then, when asked of its meaning, playfully answered: "It's for me to know and you to find out". Since then, people on the Internet have been asking about it. Besides, she mentioned in an interview that she likes using Hebrew as a secret language.
- In this interview with a Peruvian local Señora Dominica, she teases the interviewer by saying something in Quechua and not revealing what she said, so that he would become curious and learn Quechua too.