In one crucial respect, however, the Normans deliberately spurned the linguistic tradition they encountered. As we've already noted, English was highly unusual in being a written as well as spoken language. In England, not only were books and chronicles composed in the vernacular; so too were official documents - the charters, writs and diplomas issued in the name of the king. But in or around 1070 the use of English for such documents suddenly ceased.
... From 1070 onwards, therefore, all royal documents were written in Latin, a language that was familiar to both literate Normans and educated Englishmen.