Are Spanish language skills required?

No but it helps. I know minimal Spanish and I had an amazing time. There will always be someone with you who speaks English like a coordinator or someone from IVHQ but you should be fine!

Yes, at least the basics. Try a few classes on online sites like Duolingo and/or a phrase book. It will make your life much easier and minimal communication is required. Don't expect most of the people we interact with to know English, apart the volunteer organization and travel agencies, or at least it was my experience in Costa Rica. And they will appreciate the effort !