Hi,
The button is no hardcoded, it's dynamic based on the user language.
$lang = JFactory::getLanguage();
$this->button = '<div id=paypal_express_button style="width:144px; height:46px; margin-top:15px;"><a href="'.$notif.'&setExpressCheckout=1"><img src="https://fpdbs.paypal.com/dynamicimageweb?cmd=_dynamic-image&buttontype=ecshortcut&locale='.str_replace('-','_',$lang->get('tag')).'" align="left"></a></div>';
If you want to force it to a specific language you can edit the file "plugins/hikashoppayment/paypalexpress/paypalexpress.php".
Of check your Joomla configuration regarding the languages, and be sure that you have the correct tag set.