Language string in custom override not working

I'm having some dificulties recently with a template override I have created. I have made an override in the flat-template of the file month/tmpl/calendar_body.php In this override I added an extra button on the right side of "Next month". The name of this button is formatted as language override, which is available in the Joomla-template I'm using. The error suggest that this language override, named TPL_PLONS_AGENDA_LIJSTTEXT, can't be found.

This is the URL where the error message is displayed: https://www.plons.nu/agenda/maandkalender/2021/12?catids=
Attached is the file with the override of the flat template
I'm using Joomla

This is the php code in beginning of calendar_body.php

$app  = JFactory::getApplication();

$thismenu = $app->getMenu()->getActive()->link;



use Joomla\String\StringHelper;



$cfg	 = JEVConfig::getInstance();



if ($cfg->get("tooltiptype",'joomla')=='overlib'){

	JEVHelper::loadOverlib();

}



$view =  $this->getViewName();

echo $this->loadTemplate('cell' );

$eventCellClass = "EventCalendarCell_".$view;



// previous and following month names and links

$followingMonth = $this->datamodel->getFollowingMonth($this->data);

$precedingMonth = $this->datamodel->getPrecedingMonth($this->data);

This is the code that was added after the Next month button:

			<div class="lijstweergave">

		      	<?php echo "<a href='".JRoute::_($thismenu)."' title='".JText::_(TPL_PLONS_AGENDA_LIJSTTEXT)."' style='text-decoration:none;'>".JText::_(TPL_PLONS_AGENDA_LIJST)."</a>";?>

			</div>

Can somebody help me get my override back working again?