Why the other sovereigns (Jean I and Joséphine-Charlotte of Luxembourg) don't use to sign their names with "R", "GD" or "P"?
I also know HM Queen Elizabeth II of Great-Britain used to sogn "Elizabeth R".
"R" stands for the latin word Rex(king) or Regina(queen). FOr example, in Latin, The queen of England's name is Elizabetha Regina Secunda. Using the latin has come down from early times and now is just shortened with the "R". (btw: Charles will be Carolinus Rex Tertio, if he chooses the name Charles). As far as the other heads of state, I guess that using the latin form just isn't a practice that has evolved over the years, and sometimes there is not that good of a latin translation for "archduke", etc. I'm sure that it depends on the tradition of each country.
Mandatory Credit: Photo By PAUL BROWN / REX FEATURES SIGNATURE OF QUEEN SILVIA IN THE MEMORIAL BOOK SERVICE OF THANKS GIVING CELEBRATING 350 YEARS OF PEACE AND FRIENDSHIP BETWEEN THE UNITED KINGDOM AND SWEDEN AT THE SWEDISH CHURCH, LONDON, BRITAIN - 28 APR 2004 EXCLUSIVE 451205/PAB