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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.

See:

Plain English Campaign

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

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