Hi there
as for any other text in the plugin, you’ll just need to use .po and .mo files, as described here
If you already correctly translated strings in .po and you don’t see your translations applied on frontend, one of the following is the probale source of problem:
1. cached translation files under wp-content/languages/plugins: search for translation files named as the plugin in that folder, and if you can locate them, delete them
2. theme’s overridden template: if theme overrides wishlist template, and changes text domains, translations within the plugin won’t apply, and you’ll need to translate those strings from theme’s translation files