Funny, one of my favorite things about Mistborn was Spooks dialect. I found it hilarious.
Not to be rude, but sometimes trying to hold to "true" to life is a detriment to a good fable. If you are trying to hold close to how things are, then by all means do so... but it is fantasy, it is your world. If people want to talk in a way the no one on planet earth would ever talk... by all means do so.
Usually a made up language is used to give the reader just a hint at what is actually being talked about, or to hide it from them completely. If that is the case, it "sounding" realistic to me is not necessary. It would be nice, but not something I as a reader will lose any sleep over (or interest in the story for that matter.)
But then when I read a story I really care more about the plot and the characters than the fact that perhaps something "really isn't possible" in the real world. Not to pick on anyone, but in a recent review someone pointed out that it is actually very difficult to puncture straight through someones chest with a sword due to the Breastbone providing tremendous defense. Now changing it so that it sides through a rib instead is probably a good idea since it actually is more possible, but if I read a story where someone was stabbed through the chest, franky I don't really care how possible it is. I can see that in my mind, I can feel it, so what if it not terribly realistic.