Hi, Katie!
The plugin page says “This plugin is not properly prepared for localization (View detailed logs on Slack). If you would like to translate this plugin, please contact the author”. At the moment, I have only access to the readme documentation.
The Spanish WordPress community suggested that I should contact the author in order to translate the plugin completely. At the moment I will work on the translation of the available readme documentation. I guess there will be more text strings to translate that I do not have access to at the moment.
It is a great plugin: Zotero + WordPress!
Thank you for your collaboration.