The Spanish/French/Italian/German Experiment websites have free beginner lessons.
Easy Peasy All-in-one Homeschool has free Spanish lessons.
You could check the Mason’s Living Languages site for ideas of what to include and then make your own curriculum using free readers and primers from archive.org or Project Gutenberg, folksongs on YouTube, and check out board books and early readers from the library. No, it’s not like DuoLingo, but it’s free. If you want to hear the language, you could always enter sentences in Google Translate. Or listen to the Bible in your chosen language. 🙂