This would be someone adept at writing in **plain** English. 

Plain is a recognised term to describe the use of the English Language in the way you have described. 


[***Plain English Campaign***][1]

> Since 1979, we have been campaigning against gobbledygook, jargon and
> misleading public information. We have helped many government
> departments and other official organisations with their documents,
> reports and publications. We believe that everyone should have access
> to clear and concise information