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I had similar issues: I translated all email subjects in BO but when I got an email, the subject was still in english.
For some reason Prestashop did not use the translations. I do not have any idea why or was able to figure it out.

But what I was able, is to hardcode the translation in the file "lang.php" which can be found here: \themes\YOUR_TEMPLATE\mails\YOUR_LANGUAGE\
In the example below you see 2 hardcoded translations.

<?php

global $_LANGMAIL;
$_LANGMAIL = array();
$_LANGMAIL['Your order has been changed'] = 'Ihre Bestellung wurde geändert';
$_LANGMAIL['Product out of stock'] = '';
$_LANGMAIL['Product available'] = '';
$_LANGMAIL['New order : #%d - %s'] = '';
$_LANGMAIL['Stock coverage'] = '';
$_LANGMAIL['New return from order #%d - %s'] = '';
$_LANGMAIL['New credit slip regarding your order'] = 'Gutschrift für Ihre Bestellung';

?>

I hope this helps to solve your issue.