You can take some Spanish classes before arriving. I really can recommend this. I did 4 weeks of Spanish and after this amazing project. It helped me so much to get in touch with locals. Jamso Trainee is very flexible; they also offer 1 week of Spanish what I really liked.
Costa Rica itself: be relax, don't think things are like in your home country; be open to new things, then you will enjoy Costa Rica a lot!
In the morning, you start with a great typical Costa Rican food such as Gallo Pinto, Empanadas... The host also makes pancakes and cereal with fruits some days.
Afterwards, we walk to the project and the shift started at 8-11 am. After that, we go back to the volunteer house or to the beach ;)
In the afternoon, I helped sometimes as well with the shift from 3-5 pm, but it wasn't regulation to help two shifts. Like this, I had a lot of free time to discover Costa Rica.
My biggest fear is crawlies such as spiders... I was happy I didn't see any of it.
In this, Jamso Trainee did provide as well information - that we go to a tropical country and that we need to have a look that all is clean. We always followed, so I was very relaxed. I heard from people going to the rainforest that there are more spiders, but never went myself there.
During your language school, you will have cooking classes. And you know people are so happy and definitely it works, even if you will understand in the beginning just a little bit. For this, it is very good that transport is provided from the airport and to Uvita. As well without any Spanish, you can come to this project, as there are always other travelers with language skills which help - like me.