Nietzsche reminds us that all communication is 'common'.
However, this has always been seen as a *danger*; for we don't always want 'everybody' to know what we are saying.
Therefore new 'private languages' were devised; but as is the nature of these things, they too soon became 'common' and no longer private.
It was the need to create secret languages that brought about writing, which at first was a kind of secret code, and so a means of keeping a particular 'private language', secret.
How one's heart sings today, when it is said that a certain script is 'impossible' to decipher!
The decay of language is in its becoming decipherable to all.
He is Noble - i.e., he is unfathomable.