Here's a disclaimer: sticking only to Spanish platforms will probably close doors for you, because the bulk of the freelance translation market is on international sites like Upwork, Fiverr or Proz. That said, I get that you want to start out more relaxed and close to home. In Spain you've got Freelancer.es, which works decently for local projects, though the competition is brutal and prices tend to be pretty low. There's also Todocolección if you're looking to translate product descriptions, or you can contact Spanish translation agencies directly - they often subcontract work to freelance translators. That last route is usually less chaotic than open platforms.
What I'd really recommend is that you don't rule out Upwork or Fiverr, especially since that's where clients with decent budgets gather. Yeah, they're in English, but setting up a profile doesn't take much and the earnings make up for it. In the meantime, try Spanish sites to build experience and get your first gigs. Build a small portfolio with real translations (even if the first ones are pro bono just to have samples), be specific about what kinds of translations you're good at (technical, literary, legal, marketing), and ask for references. On the practical side, use tools like Mémrise or free CAT tools if the volume picks up. The most important advice is don't waste time waiting for ideal clients to show up: spend a couple of months on whichever platform you pick, see what works, then expand. The online translation market is like that - you've gotta be where people are willing to pay.