You can also find people here in Costa Rica that will be willing to stand as witnesses, such as hotel employees or new friends. In order to register the wedding back home, you will first have to register the marriage at the Civil Registry of Costa Rica. You will then need both a certificate from your embassy authenticating all signatures, and official translations on all documents. This procedure might take up to 2 months, but the same lawyer handling the wedding can do all this paper work for you, and mail it to your home address at your convenience. A religious celebration could be more complicated, since most Catholic priests encourage couples to take a course on spiritual guidance before marriage.
Bilingual priests are scarce, specially in the back country.
Your best choice will be to arrange with the priest serving mass at the hotel on weekends. The town where you stay at will have its own church also, and you could arrange a wedding service with them. All in all, tourists have had religious weddings here in Costa Rica, and instead of listening to the music from a piano, they heard a beautiful melody of waves, ocean and wind, and nature overwhelmed their improvised chapel on the beach. So, don't let the paper work and legal talk put you down. Make arrangements at the hotel of your choice, and follow your heart.