'Entendre' in French means both 'to hear' and 'to understand'. As a pun, it does not have the force that an Anglophone might think, just as 'I see' in English means both 'I see' and 'I understand', and would lack metaphorical strength as a pun because its use in the second sense has become a cliché.
With a bit of freedom one might translate the Sade quote as: 'I speak only to those who are able to hear, for they alone may read me without fear.'