How do you try to type Cyrillic on Windows?  Do you use the righthand Alt key (because the Cyrillic characters are on the third level)?  Or do you switch keyboard layouts when you want to switch from writing English to writing some East-European language?

Are you using a UTF-8 locale?  That is, if you paste the following string into nano, "色は匂へど散りぬる -- Кирилица в днешно време -- მარგალური", does it display the three different scripts correctly?  Or do you get a bunch of Unknown-Command warnings instead?

Hm...  So it wasn't the keyrebinding stuff that broke Cyrillic, because that arrived in 2.1.0.

Please try to find the latest version after 1.2.4 that still works with a Cyrillic keyboard layout.  Start somewhere in the middle between 1.2.4 and 2.0.3, and in that way narrow it down to a single version change.

If you cannot type anything at all, then please provide the requested output file from a compilation with --enable-dubug.

And please answer the question: was there any earlier version where Cyrillic typing did work?

Did any previous version of nano /do/ work with Cyrillic letters on Windows?

Do you mean you cannot type anything at all?  Not even plain Cyrillic text into an empty buffer?  Or is it just the Alt+<key> combinations that don't work?  Because that is what happens for me on Linux.  When I choose a Bulgarian or Ukrainian layout, for example, I can type stuff, I can save it, I can search for things -- everything that uses Ctrl+<key combinations works.  It is just all the Alt+<key> combinations that produce the "Unknown Command" message.

Trying to bind a Cyrillic character to a function in the nanorc file doesn't work (something like 'bind M-Ч nohelp main').

If you are able to compile nano yourself on Windows, then please ./configure it with --enable-debug, run it as 'src/nano 2>TRAIL' (or however that works on Windows), set your Cyrillic layout, and type the equivalent of Q W E, ^A ^S ^D, Alt+Z Alt+X Alt+C, and attach the resulting TRAIL file here.