would anybody please tell me is that sentence an idiom and what does that mean?
It appears in an article on linguistic issues and the full sentence is:
“English, for all its confusing spelling and exceptions – if a baker bakes, what does a grocer do? – has a relatively simple grammar”
Try Googling the phrase Vuongdieulinh, you’ll get a reply.
sorry but I found nothing. 
I can understand the meaning but what I confuse is the aim of the author. why did he add this idiom to his sentence
What is the idiom you are talking about?
This is a quotation by Lupyan.
A grocer grocers. That is his role in life. To sell a variety of merchandise.
yep,
the butcher butches ::smile
The most simple answer seems always the best.