Unless you're a linguist like Tolkein, I suppose the simplest way would be to pirate the grammatical structure of an existing language and just make up word substitutions. In most cases I think you'd have to be careful to match the "feel" of the language with the races that speak it though. It would be jarring to read an eloquent-sounding language spoken by a race of beings that were barbaric/barely more than animals. With some exceptional creativity you might make something like that work but it wouldn't be easy. As a related aside, throwing some naming conventions (rules) into a cultural backdrop can be a nice way to add depth to a story and reveal something about the characters.