I speak Russian, but I've never used Russian translation in C3. After reading this post I thought "how bad can it be?" and switched to Russian. Turns out, really bad..
Most descriptions of events/actions/plugins appear to be made with Google Translate and there are tons of mistakes in Russian text, to the point that it's often incomprehensible.
However, for me the worst part is expressions. Lots of Russian translations of expressions are complete gibberish and make zero sense.. Also, the program doesn't allow you to type English expressions (so you have to type "определить" instead of "int", which is a totally wrong word). I have no idea how people can edit expressions in Russian, it must be a torturous experience. I would suggest leaving expressions as they are, in English form.