I have a problem during search Cyrillic texts inside documents. Find function works always like Case sensitive checkbox is on. So, I can't find "Тест" by substring "тест", but I found it well by "Тест" substring.
The language Windows itself uses should be the best indicator. For example, File Explorer or the Start Menu.
Another thing to check: If you try the same search in File Explorer, does it find the files OK?
It's possible we're not using the locale for the case-insensitive search, but it's also possible the locale itself doesn't consider the two strings to be the same if it's not set to Cyrillic, or that the Word document IFilter (part of Office) that turns doc files into plain text for search tools isn't handling the characters correctly. (I haven't had a chance to configure a machine for Cyrillic to check yet.)