Maybe the aliens do not talk is because they are not trained to put words to their thought. Communicating through speech is more than just selecting accurate words. You must have a large reference on how the word is acceptingly used.  You have to choose you words in a similar manner as the person listening to you would. You have to translate thought into words. It's not as easy as just learning the dictionary. You have to be able to think on a similar level.

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