http://bugs.winehq.org/show_bug.cgi?id=18466
Summary: Wine does not support genitive forms of month names Product: Wine Version: 1.1.21 Platform: Other OS/Version: All Status: UNCONFIRMED Severity: trivial Priority: P2 Component: -unknown AssignedTo: wine-bugs@winehq.org ReportedBy: tarasov.igor@gmail.com
MSDN: "Some languages, such as Finnish, German, Polish, and Russian, have several noun forms. If you plan to use localized names provided by the system, have a linguist check to make sure you are using them in the right context. Windows carries both the nominative and genitive forms of Polish and Russian month names; the form changes depending on the month name's position in the string relative to the day name. Windows returns both forms in a single string separated by a null value. The system carries only one form of each month name or day name for all other languages."
http://msdn.microsoft.com/en-us/library/cc194815.aspx
Wine ships only nominative forms for these languages. So, applications that rely on this, render dates not as "14 мая 2009", but as "14 Май 2009".
Side note: first letter in genitive form is lower-case, as opposed to to upper-case for nominative form.