Especially in a Japanese windows, after changing the system language from Japanese to Bulgarian (or any other Cyrillic language), the letters used for some system dialogs would be displayed in full width font making it at times almost unreadable. There is no obvious place to change those fonts, and the only help is in the registry itself. Navigate to this key in the registry:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes

And change the values for MS Shell Dlg and MS Shell Dlg 2 to a non-Japanese font (the default MS Sans Serif and Tahoma are actually pretty good). You can also try Arial Unicode MS if you still care about Japanese characters. There is a more detailed explanation at this page.