There is no way to convert the resulting seed back to the original mnemonic.
As stated in the BIP 39 proposal:
To create a binary seed from the mnemonic, we use the PBKDF2 function
with a mnemonic sentence (in UTF-8 NFKD) used as the password and the
string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt.
The seed (and optional ...