For future readers who are, like myself, new to Strapi (v3), plugin translation will work by prepending the plugin name, like so:
<FormattedMessage id={${pluginId}.CustomString.substring} />
For future readers who are, like myself, new to Strapi (v3), plugin translation will work by prepending the plugin name, like so:
<FormattedMessage id={${pluginId}.CustomString.substring} />