Cyrillic alphabet characters in JSON response

Well, since no answers have appeared in this topic, I'd like to give a solution here in case someone else needs it later.

You simply need to open the file SoapUI-5.3.0.vmoptions (or whatever version you are using) with a text editor and add 
-Dfile.encoding=UTF8

on a new line, and save it.

The file can be found at the bin folder of your SoapUI application. For instance, I have mine located at C:\Program Files\SmartBear\SoapUI-5.3.0\bin

After this is done, restart your SoapUI and everything should work fine.