Reproduced in text-editors based on Qt.

1. On WinXP install some unicode OpenType font (for exapmle OldStandard-Regular.otf freely avaliable from http://www.thessalonica.org.ru/en/fonts-download.html)
2. Select installed font in QT text editor settings
3. Open some multilanguage file in TexWorks editor
4. Look at the result - it is rendered by 2 differnt fonts, although selected OpenType font has all needed symbols to show file correctly.

All file content should by shown using selected OpenType font. But i see at least 2 fonts. English letters, digits and symbols are shown using selected font, but russian (and other) symbols are shown using some other font (looks like "MS Shell Dlg" or something similar). Other text editor (non QT-based) renders this file normal (on the same system).