By default, WPML will not scan or load the translation files into the database. So, you will not be able to find these strings on the “String Translation page”.
Themes and plugins usually come with their own translations for the texts for the user interface as well as their front-end output. It will store the translations inside translation files.
To translate or edit the strings, you first need to scan the translation files of the related theme or a plugin. For that, navigate to the WPML -> Theme and Plugin Localization.
Select the required plugins to get the strings under the “Strings in the plugins” section. You need to select the Salesforce Customer Portal plugins and click on the Scan selected plugins for string button.
Once you get the Scanning Results, you can now translate the SF customer portal strings from the String Translation menu.