I'm a Chinese user and got the same issue. I found the thread
http://goo.gl/uqX8 in hi-pda which localized ROM 1.05d. I compared 1.05d with 1.05f ROM and found the structure are almost the same. So I extracted both roms and copy fonts from 1.05d to 1.05f. Burned the 1.05f back to img. Then I flash the new ROM to my PRS-600.
After that, It works. Although the localize is not perfect. The main menu is still in English. But, anyway, I got English dictionary and can read Chinese.
I attached the new img here. You should replace the img file from 1.05f with this one. The location is PRS600.Firmware.Reload.Package\test-flash.