Hi, normally the interface language is triggered by the learner in the profile settings, you're right. At initial login the interface language is set to English, so each learner than needs to manually set it to his/her preference.
In the Admin role, settings menu, Basic Info, there is "country" and "locale" setting, which you could change to Spain and Spanish, but not sure if that automatically changes the default interface language to Spanish.
and of course this would then be applicable to every user in the system, not only for your specific spanish learners.
Good point though, you should be able to set the initial language of each learner through CSV uploaded user data, or create an initial login popup window where the first question is asked to set your default language.
This will prevent them to not understanding the interface at fist login, which would give a more positive initial UX