For installing 3.2.1 (the firmware of 3rd generation kindles) on your kindle DXG and use the Cyrilic fonts natively on your kindle DXG, have a look at this thread:
https://www.mobileread.com/forums/sho...d.php?t=127969
For font hacks for 2.5.8 for kindle DXG have a look at here:
https://www.mobileread.com/forums/showthread.php?t=88004