Language will not be a problem. Get a hold of written directions of where you are going and a good map and have someone circle where you will be going on the map. Then just ask, ask and ask along the way. Dominicans are world known for being friendly and helpful. And the likelihood of someone knowing English is very high.
We have traveled worldwide to countries where we do not know the language -- Japan, Italy, France and Greece, for example, and it makes a world of difference to go by car, or by public transport, not taxis.
Just take this bit of advice: Ask when you think you are going in the wrong direction, and also ask when you think you are headed in the right direction.
Go for it. Renting a car is the best way to go. Just do as we do when we travel... even when renting in the US... we take out all the insurance one can take so in case of any problem, the problem is left with the insurance company, and rent with a reputable company.
Of course, if you do get lost, you can always hire a motoconcho to show the way to where you are going. Or any friendly driver on the road may head you in the right direction.
Going by car you can explore Cofresi, Luperon (La Isabela), Playa Dorada, Sosua, Cabarete and then head out to Playa Grande all on your own. See info on these destinations in the travel section.
http://dr1.com/travel/index.shtml