@otto,
To perform translation, the very first step is to check your site language from wp-admin > Settings > General >
http://prntscr.com/ta8xjd
So in loco translate, I will choose English language if I want to translate something in any plugin/addon
In your case, if your site language is Spanish. Then add the translations in Spanish Language.
In case you will not find any language then you can create a new one
Rest of the steps are same:
https://wplms.io/support/knowledge-base/translate-using-loco-translate-plugin/