WooCommerce is plugin on its own which means that you need to translate WooCommerce strings in the WooCommerce plugin directory. Go to wp-content/plugins/woocommerce/i18n/languages/ and translate the woocommerce.pot file (search for the words you want to translate). Then add the po and mo files to the same directory (wp-content/plugins/woocommerce/i18n/languages/) with the correct file names.

Keep a copy of the translation files on your computer, since these files can be overwritten on future WooCommerce plugin updates.