Most of the language strings for PublishPress plugins are in the main language file.
So the first place to look, if you have installed one of the PublishPress plugins, is in this folder:
/wp-content/plugins/your-installed-plugin-name/languages/However, some strings you see on your site are in different locations and this may result in some missing strings. For example, PublishPress uses some strings from the WordPress core. If you want PublishPress to be in Spanish, make sure you have Spanish installed for the WordPress core.
If you can't find a particular language string, contact us and we'll be happy to help.