Solution:
Check your code but it work in snippet
jQuery(document).ready(function( $ ){
var content = 'ΟΛΑ';
$('.qodef-pl-filter-holder ul li.qodef-pl-current span').html(content);
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="qodef-pl-filter-holder">
<ul>
<li class="qodef-pl-current"><span>Show All</span></li>
</ul>
</div>