Perhaps one of the hardest parts of creating a culture whole cloth is developing their language. While one could go the route that the esteemed J.R.R. Tolkien did and develop their language from the very basics of syllables and lettering, it is equally difficult to describe how people within your newly minted world speak on a casual basis. In a fantasy world that is in no way connected with Earth, one finds themselves adrift with out the expressions that anchor casual conversation. To say that someone is "against the clock" when they are attempting to complete something as fast as possible by a deadline does not make sense in a world with out clocks.
There are many other little expressions that we use in our daily lives that we pay no mind to. These idioms are more then linguistic habits. They are a cultural shorthand for ideas that, at times, are strictly ingrained in said culture. A statement such as "a ball-park figure" become nonsensical when removed from the culture they originated in. Thus, authors who are creating a world that lacks these cultural elements are forced to develop new idioms that take place within the context of their world. This is not an easy task.
Idioms express many things and have shades of meaning that can be different given the intonation of the statement, the context of it, or any number of additional secondary meanings that are divergent from the original idiom and the meanings of the words that create it. It is, however, not a task that is insurmountable. With a little planning and careful consideration, one can transform an expression that is commonly used in our world to something for the fantasy world but still recognizable.
For example, within the kingdom of Evandar, an expression that is comparable to 'devil take it' is 'darkness take it'. The statement is used in the same context that 'devil take it' would have been in an Earthly conversation and carries much of the same weight. After a little exploration, the expression 'darkness take it' is revealed to speak of the abode of the god of evil and says that this deity should lay claim to that which this pejorative is declared against. Where the difference between the two expressions lies in a single word, it changes considerably and takes on a different dimension within the fantasy world.
It is a time consuming task but it adds greater depth of realism to the fantasy world. Nothing is as jarring as encountering anachronism in a historical novel or something that is blatantly of Earth in a novel that is entirely focused on a world that has no contact with ours. I exhort you, my fellow writers, to dive into your world and take the time to build idioms. While you may not be building an entirely new language, the idioms become an important part of creating the suspension of disbelief that a truly great novel has. Our readers deserve nothing less.