sigh - newbies ... ;P (just joking)
so -
do the changes as below
file templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default.php around line 921 change to
<?php if (!empty($customTab1_title)) {?>
<div id="customTab2" class="tabcontent" style="display:none;">
<?php echo JText::_('PUREMART_SHIP_TAB');
#echo $params->get('productdetails_customTab1_content');; ?>
</div>
<?php } ?>
you can also use this technique in the tab titles - around line 823
<?php $customTab1_title = $params->get('productdetails_customTab1_title');
if (!empty($customTab1_title)) {?>
<li><a href="javascript:void(0)" rel="#customTab1">
<span>
<?php
echo JText::_('PUREMART_SHIP_TAB_TITLE');
#echo $customTab1_title;
?>
</span></a></li>
<?php } ?>
so now your using these two language variables as the tab title and tab content
PUREMART_SHIP_TAB_TITLE
PUREMART_SHIP_TAB
now all you have to do is set the content of these variables in any lang ini ..
probably for the moment it's easiest to use Joomla's native lang overides - see screens - do for each language
Cheers