From what I remember, when you install Boroda there is a dir created in int memory root called
/test-startup. It contains a subdir
/test-startup/fonts which contains 3 customised font files
tt0003m_.ttf (sans-serif),
tt0011m_.ttf (serif) and
tt0419m_.ttf (monospace) which will be used to render LRF/RTF/TXT files (not EPUB). I'm not sure, but it's possible, that the sans-serif font is also used to render the Sony menus.
There is also a file called
/test-startup/startup.sh
With PRS+ you keep the
/test-startup directory in the same place but the file called
startup.sh should be renamed as
prsp.sh and moved to
/database/system/PRSPlus/prsp.sh
When you do a full reboot, after installing PRS+ I think you should get the same functionality as Boroda as long as the 3 tt*.ttf files contain all the special characters you will need for your Russian (or any other language) books.
This
SonyFont utility was developed by forum member pepak to help you create customised tt*.ttf files from any font you choose.
Note: I'm not sure if you need to be careful about the size of these font files.
Customising EPUB fonts is done completely differently, using selectable CSS files. If you wish to use the same 3 tt*.ttf font files you can use the FontsAsInLRF.css file supplied with PRS+. However, you have much more flexibility with EPUB and can use any .ttf or .otf font which takes your fancy, as long as it contains all the necessary special chars for your chosen language.