Welcome to Eorzea!

There are only four languages covered in game text/dialogue (Japanese, English, French and German) - unfortunately there is no Spanish language version available I'm sorry to say. Try setting all four languages for Duty Finder as that may help. You can also instead of using Duty Finder to try forming a party in Party Finder and let people know in the message that you need help and speak Spanish - usually people will offer to help if you're humble.

If you're trying to enter a duty which is pretty old then sadly a lot of people probably aren't interested in doing them at the moment (which is Duty Finder's failing), but still, don't give up! Use Party Finder instead in these situations, even if it might take a while to form a party to enter, it's still often better to do it than relying on Duty Finder to form your party for you.