You will need to run these links through google translate as they are in russian. The basic goal of these hacks are to remove dutch and replace it with russian as well as add the clock to the lower right corner. English is still supported and defaults to that unless you tell it otherwise.
Russification Sony Reader PRS-900 (ver. boroda)
Russification Sony Reader PRS-900 (ver. amutin)
** PLEASE BE AWARE THAT INSTALLING THESE HACKS MAY BRICK YOUR READER AND I TAKE NO RESPONSIBILITY IF THAT OCCURS **