I have a small script which works just fine it displays a calendar, however someone downloaded it and wants to use it in Russian, here is a link to a demo site the calendar is on the left, if you notice the calendar is displayed just fine at first, however if you skip a month or a year then on update the calendar will display unknown symbols, is there something special that needs to be done to the file in order for it to work?

The problem is that the day initials are sent on post

The calendar works fine if language is set to English.

Edit: just wanted to add that the day initials are stored in a JavaScript variable
var dayInitials99 = "ПВСЧПСВ"