thp-wup-translation-using-gettext.php

<?php
function thp_wup_custom_change_strings( $translated_text, $text, $domain ) {
	
	if ($domain == 'very-simple-woocommerce-upsell-popup') {
		
		switch ($translated_text) {
			case 'The item has been added to cart. Thank you!' :
				$translated_text = __( 'Der Artikel wurde in den Warenkorb gelegt. Vielen Dank!', 'very-simple-woocommerce-upsell-popup' );
				break;
			case 'Would you also be interested in this item?':
				$translated_text = __('Würden Sie sich auch für diesen Artikel interessieren?', 'very-simple-woocommerce-upsell-popup');
				break;
		}
	}

	return $translated_text;
}
add_filter( 'gettext', 'thp_wup_custom_change_strings', 20, 3 );