Defining format-preserving encryption for natural language Assume we have plaintext in natural language. Can we use format-preserving encryption in such a way that the produced ciphertext cannot be distinguished from natural language?
The below is assuming a natural language is the one that has sentences from a well defined dictionary for that language (ex: oxford dictionary for english ).

We can do this with the standard approach for FPE i.e $rank-encrypt-derank$.
Build a $key-value$ pair map of all the  words in the language of preference  Where $key$ is the number and $value$ is the word. {Say from $1$ to $n$ }. $n$ being maximum number of words in dictionary.


Rank : For each word find the key which is a number
Encrypt: Apply the FPE algorithm of your choice (FF-1,2,3 OR FNR) or even simple fiestel network. 
Derank the ciphertext of FPE algorithm which is a number by finding the respective word in the map. So the deranked word would be actual ciphertext


Decryption is similar .