General translations:
* Langage secret: This is the most straightforward and common translation, literally meaning "secret language".
* Langue secrète: Similar to "langage secret", this translates to "secret tongue".
More specific translations:
* Code secret: This emphasizes the use of a coded system for communication.
* Jargon secret: This implies a specialized language understood only by a particular group, often with a hidden meaning.
* Langue des signes secrète: This specifically refers to a secret sign language.
Other options:
* Parole secrète: This translates to "secret word" and could be used in a context where the secret language is a single word or phrase.
* Slang secret: This refers to a secret slang used by a specific group.
The best translation for you will depend on the specific context in which you are using the phrase.