Firstly, you should work on a Windows machine in Russian
(wether it is a virtual system or not). A machine with multilingual
compatibility might work, but I am not sure about that. Then you
can work with Robohelp normally. Characters will not show
correctly, but you can fix that later.
Once you have compiled your project with Robohelp, you must
open all the HTM files with a text editor (such as Ultraedit,
editplus,etc...) to search an replace the charset in all files. You
should replace 1252 with 1251 (which is the Charset for Russian and
Bulgarian languages).
Then you should use Microsoft's Html Help Workshop (availabe
free at Microsoft's website) to do the final compilation with that.
You don't need to make any other changes just open the project and
compile. Your project in Russian will be ready after that. Of
course, you can do everything with Htlm Help Workshop, but it does
not have a wysiwyg editor, so you'd have to work on the
code.