An idiom is an expression whose meaning is not compositional—that is, whose meaning does not follow from the meaning of the individual words of which it is composed. For example, the English phrase to kick the bucket means to die. A listener knowing the meaning of kick and bucket will not thereby be able to predict that the expression can mean to die. Idioms are often, though perhaps not universally, classified as figures of speech.
Slang is informal language consisting of words and expressions that are not considered appropriate for formal occasions; often vituperative or vulgar; "their speech was full of slang expressions"
at different culture, different kinds of idiom develops, so that is why English idioms are so different from Chinese成語. if u say horse horse tiger tiger instead of to palter with something, no English speaker would understand you as they are from a culture different from chinese.