Is it ethical to publish a paper about porting an existing software package to another programming language?
Your paper must clearly delineate what is your original contribution and what is the same as, or based on, the previous package. So if you write about your Corpus class, for example, you must say that it is modeled after the R package's Corpus class. Otherwise it is "cheating," as you put it.

(It's also not clear to me that a straightforward port of an existing package would be considered a novel research contribution by most venues for publication. But that's an entirely different question.)