Ah, Translation. That makes sense.
"to improve your English" was not meant as a slur on your English writing skills. A better phrase would have been "to improve your grasp of obscure slang in English." It is the kind of piece that most native speakers tend to glide over without working too hard on the details.
I admit though that for me part of the fun of Nero Wolfe and other writers of a previous generation is the need to keep a dictionary handy for new words.