If you go to the british flag, then you will find yourself on the english language pages. On the left side of the screen, you will see a place where it says Find your route from one address to another in Paris and its suburbs. Well, click there, and you will find a very handy search function, where you can put the address you are leaving (Charles de Gaulle Airport, in your case), where are you going (Cergy, and it will ask you to be more specific), the dates, the time,... You can type in a street name and a number... It is really great (and useful, I have already tried for itineraries inside Paris). It gives you a map of your destination, and your itinerary.

To buy your tickets, there is a ticket counter inside the airport arrivals hall. I don´t think you should have problems with the language, but just in case, write down in a paper your destination.

Another thing, I wouldn´t recommend to jump into the RER (and a change of train) if you are carrying a lot of luggage. It is not very comfortable to move around if you are loaded with bags (I´ve done it, and I am trying not to do it again).

Rgds, Covadonga in Bilbao