What does the Diplomat do?

The diplomat position in the alliance (as with the Guardian, Recruiter and Overseer) are just special names R4 level roles that members can be assigned to. I believe at the moment they do not come with any special buffs.

The diplomat may be asked to be in charge of messaging other alliances to call truces, negotiate surrenders call off attacks etc. If the alliance wants that to be done. There is no in game requirement to have a diplomat though.