In short, language is more complex than just getting the words right. That would be like trying to walk without footing. You'd miss really important knowledge to communicate effectively. (However, it could work on the internet)
My wife's native language is Chinese and her experience has made me keenly aware of the cultural context of communication. IMO the part that most people overlook is the importance of humor in communication. Observe a casual conversation and pay attention to the number of jokes and witticisms that are part of the exchange. Jokes are almost always rooted in a cultural context, and the sentence without understanding of the context doesn't provide enough. If an American says "REDRUM REDRUM", most other Americans will get the reference. However, a translator will likely just bork on it. Speaking of Chinese, they have numerous 4-word phrases that have no literal meaning, but have understood meanings that are often quite sophisticated. For example "Horse Sky Fall Frog" could be the English translation of a meaningful 4-word phrase. Of course these translators will be more useful with every passing year, but I think it will be quite some time before native languages succumb to a universal interpreter. On top of that, once you have an interpreter, people are going to start messing with it as a means of communication in of itself.