The default language plugin is just the built in English language plugin. CB doesn't set what language is loaded by default. That's entirely handled by Joomla within Extensions > Languages. So to have Spanish as the default you need to have a Spanish Joomla and CB language plugin installed then set it as default within Joomla.

As for overriding language strings within CB please see the below blog post.

www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived

We do not provide translations. We simply package them for your benefit. All translations are a community effort. It appears that the Spanish language files are incomplete as shown below on our transifex.

www.transifex.com/joomlapolis/cb/

Try adding an override for it and translating it there if necessary.

There's a language key and string finder directly below the language overrides usage. You can search for language strings using it. You can also enable language debugging in CB > Configuration > Integrations that can show a table at the bottom of the page of any language strings being used on the currently viewed page.