Written material and Written language

Written material noun - The work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect).

Written language and written material are semantically related. In some cases you can use "Written language" instead a noun phrase "Written material".

Written language noun - Communication by means of written symbols (either printed or handwritten).

Written material and written language are semantically related. Sometimes you can replace phrase "Written material" with "Written language", this phrases are similar.