With all the fascination for the achievements of the modern technologies the best computer is still the human brain so far . We learn all the rules and exceptions and styles of our language, we recognize damaged (on any level: Lexis, Morphology, Syntax) phrases and reconstruct them, we build up our experience the way no computer can compete with the human mind. Yes, we are imperfect and irrational - that's why we are more flexible and unpredictable, and so is our language. Computers work on the basis of logic (remember the books of A.Asimov with his constant reminding that robots are slaves to the logic, while the humans are rational). Computer programs become more and more complicated, but computers will never create anything by their own initiative or learn from their mistakes unless their software get updated.
The indirect proof to this statement are the problems of highly functional people diagnosed with autism. Their brain functions much more like a computer than a brain of an average person. They understand world much better through the symbols and they have problems with speech, fantasy and making choices (well between a green jacket and a red one when both are of the same size and for the same weather - so choosing the colour which "looks better on you" becomes an act of torture for a person with autism). They have troubles with making an ordinary drawing creating their own images and not reproducing the patterns they saw before etc. They shut down in themselves because their brain fails to deal with our weird world - well, that's a sketch, not the whole picture, of course!
But that's how the translator program works: the Lexis is downloaded, the possible word combinations are downloaded, the possible Syntactic constructions are downloaded - and the program fits the real text in all proposed patterns. By imperfection of the translation you can see everything what is overlooked or there is no way "to explain" it to the computer. Somebody mentioned here the conjugations are missing - well, there are conjugations in English~Russian Google, but it fails all the time to translate back the same phrase properly:
ты идёшь в школу (human Rus.) - you go to school (google Eng.) - Вы ходили в школу (google Rus.) . Google turned The Present Tense int The Past Tense in such a simple sentence.
It's all about programming and difference between what is logic, what is rational and what is real life.