This sounds like a simple thing right? Just set the Locale of the SPWeb object to the locale you are interested in. Not so fast – this does not work, it will change the Locale but not things like the calendar being used. What you have to do is to create a new instance of SPRegionalSettings and set a few properties like LocaleId, CalendarType and Time24.

CultureInfo ci = new CultureInfo(your culture);

SPRegionalSettings rs = new SPRegionalSettings(web, false);
rs.LocaleId = (uint)ci.LCID;
rs.Time24 = rs.GetDefaultTime24((uint)ci.LCID);
rs.CalendarType = (short)rs.GetDefaultCalendarType(ci.LCID);
web.RegionalSettings = rs;
web.Update();