Datepicker: Get selectedDay from data-date instead of element contents

This makes datepicker work fine even if translation software modifies texts on the page.

Closes gh-1943

ui/widgets/datepicker.js

@@ -1080,7 +1080,7 @@ $.extend( Datepicker.prototype, {
 		}
 
 		inst = this._getInst( target[ 0 ] );
-		inst.selectedDay = inst.currentDay = $( "a", td ).html();
+		inst.selectedDay = inst.currentDay = parseInt( $( "a", td ).attr( "data-date" ) );
 		inst.selectedMonth = inst.currentMonth = month;
 		inst.selectedYear = inst.currentYear = year;
 		this._selectDate( id, this._formatDate( inst,
@@ -1932,6 +1932,7 @@ $.extend( Datepicker.prototype, {
 							( printDate.getTime() === currentDate.getTime() ? " ui-state-active" : "" ) + // highlight selected day
 							( otherMonth ? " ui-priority-secondary" : "" ) + // distinguish dates from other months
 							"' href='#' aria-current='" + ( printDate.getTime() === currentDate.getTime() ? "true" : "false" ) + // mark date as selected for screen reader
+							"' data-date='" + printDate.getDate() + // store date as data
 							"'>" + printDate.getDate() + "</a>" ) ) + "</td>"; // display selectable date
 						printDate.setDate( printDate.getDate() + 1 );
 						printDate = this._daylightSavingAdjust( printDate );