If you're fluent in Spanish, the tough-o-meter rank comes way down. If you're not, it can be a bit of work to follow exactly what's being said all the time. That doesn't mean you can't follow, though. Anaya does a great job of making the events of the novel clear. You still might want to grab a handy Spanish-to-English dictionary or find your favorite translation site, though. Or better yet, why not use this is an opportunity to learn a new language?
Okay, maybe that's a tall order. But hey, this book's worth it. And regardless of your linguistic skills, you'll be glad you cracked its spine. It's about a young person dealing with some of the pain of growing up, and everybody can relate to that. Plus there's magic, and magic is cool.