After I've added some highlight words to my HL list (like "Tic"), I found out that they work as expected (i.e. mIRC doesn't make a difference between Tic and TIC, and this is a correct behaviour). So I added some words written in Russian expecting the same, but I failed my expectations.

In short: please do something about it so highlight words written in cyrillic become case-insensitive. I do realize that tolower()/toupper() typically work with latin letters only and may waste the output, but maybe there is a way. And no, I don't have "Case sensitive" box checked.

This has to do with the fact that since 7.x (Unicode), mIRC's case-insensitive equality test is broken. The Unicode standard demands that default caseless matching should use full case folding (§3.13). mIRC currently does not do that.