Update HighchartsWidget.php

The 'lang' Option can only be set before the instantion of the object. see: http://api.highcharts.com/highcharts#lang

this change enables you to set the lang property for example with:
$this->Widget('ext.highcharts.HighchartsWidget', array(
'setupOptions'=> array(
		'lang' => array(
			'decimalPoint' => ',',
			'thousandsSep' => '.',
			'loading' => 'Lade...',
			'months' => Yii::app()->locale->getMonthNames('wide'),
            'weekdays' => Yii::app()->locale->getWeekDayNames('wide'),
            'shortMonths' => Yii::app()->locale->getMonthNames('abbreviated')
		)
	),
);