Parece que ya he conseguido solucionar el problema.
No se el motivo, pero en mi archivo de traducción, faltaban las siguientes lineas.
Ruta: modules/blockcart/translation/es.php
$_MODULE['<{blockcart}prestashop>blockcart_98b3009e61879600839e1ee486bb3282'] = 'Close window';
$_MODULE['<{blockcart}prestashop>blockcart_544c3bd0eac526113a9c66542be1e5bc'] = 'Product successfully added to your shopping cart';
$_MODULE['<{blockcart}prestashop>blockcart_694e8d1f2ee056f98ee488bdc4982d73'] = 'Quantity';
$_MODULE['<{blockcart}prestashop>blockcart_e5694b7726ceaf2f057e5f06cf86209e'] = 'There are [1]%d[/1] items in your cart.';
$_MODULE['<{blockcart}prestashop>blockcart_fc86c43dbffcadc193832a310f7a444a'] = 'There is 1 item in your cart.';
$_MODULE['<{blockcart}prestashop>blockcart_db205f01b4fd580fb5daa9072d96849d'] = 'Total products';
$_MODULE['<{blockcart}prestashop>blockcart_21034ae6d01a83e702839a72ba8a77b0'] = '(tax excl.)';
$_MODULE['<{blockcart}prestashop>blockcart_1f87346a16cf80c372065de3c54c86d9'] = '(tax incl.)';
$_MODULE['<{blockcart}prestashop>blockcart_f4e8b53a114e5a17d051ab84d326cae5'] = 'Total shipping';
$_MODULE['<{blockcart}prestashop>blockcart_300225ee958b6350abc51805dab83c24'] = 'Continue shopping';
$_MODULE['<{blockcart}prestashop>blockcart_7e0bf6d67701868aac3116ade8fea957'] = 'Proceed to checkout';
Si no recuerdo mal, cuando hice la instalación limpia, funcionaba correctamente, no se el por que ahora no funciona.
Espero que sirva de ayuda. Un saludo