Hi Boris,
It's defined in this file: /templates/ja_teline_v/tpls/blocks/off-canvas.php
I recommend to change the code of this file to:
<?php
defined('_JEXEC') or die;
use Joomla\CMS\Language\Text;
?>
<?php
if (!$this->getParam('addon_offcanvas_enable')) return ;
?>
<button class="btn btn-default off-canvas-toggle" type="button" data-pos="left" data-nav="#t3-off-canvas" data-effect="<?php echo $this->getParam('addon_offcanvas_effect', 'off-canvas-effect-4') ?>">
<i class="fa fa-bars"></i>
</button>
<div id="t3-off-canvas" class="t3-off-canvas">
<div class="t3-off-canvas-header">
<h2 class="t3-off-canvas-header-title"><?php echo Text::_('OFFCANVAS_TITLE'); ?></h2>
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
</div>
<div class="t3-off-canvas-body">
<jdoc:include type="modules" name="<?php $this->_p('off-canvas') ?>" style="T3Xhtml" />
</div>
</div>
Then add override for this language constant: OFFCANVAS_TITLE
You can check out this document for how to override: https://docs.joomla.org/Help4.x:Languages:_Overrides