The literature of England is literature written in what is now England, or by English writers. It consists mainly of English literature - i.e. literature written in the English language - but there are important examples of literature from England written in other languages.
English literature written in other languages needs to have been written in the country of England.
The original text says that English literature can either be in English or another language but must have been written by and English writer or have been written in England.