Dear ,
The English Department of University would like to extend to you an invitation to be our guest speaker at the annual conference to be held in the department meeting room at X o 'clock (Monday) on -- .
As you know, the Foreign Language Department of X University is interested in literature. You are familiar with the field and your views will be of great interest to us.
We will inform you of the details later, but would appreciate your prompt reply so that arrangements can be made.
Department of Foreign Languages, University