By default, Mathematica 9 and 10 use Gill Sans MT fonts for headings of title/subtitle/section/subsection/subsubsection. Unfortunately, Gill Sans MT fonts have no Cyrillic letters, so that all headings in my old notebooks have become corrupted. I found one remedy. After uninstalling Gill Sans fonts from OS, Mathematica substitutes the absent fonts with something else which turns out to have Cyrillic letters.
Is there a less radical solution?
Setting individual stylesheets for every notebooks (as explained here) is not an option for me.