Don't worry! It's a piece of cake, you speak English and you are not going to Japan or China where there is a completely different alphabet
The word train is spelled the same in French as it is in English and it means the same! How easy is that ;-)
Follow the signs from the terminal to the station and you will be fine. If you haven't looked it up before you arrive, which I get as you don't know exactly how long it takes from terminal to station, waiting for luggage etc, there is free wifi at the airport and the train station. Use Googlemaps, look at the stations inbetween so you can keep track of going in the right direction.
You boarded and you realize it's the wrong train. What would you do when that would happen in the US, even when you are not accustomed to public transportation? Most likely you ask the people standing next to you or if you feel independent, you get out at the next stop and take the train in the opposite direction. You wouldn't stay seated till the end of the line and wait till someone would get you and tell you, you are in the wrong train ;-)
Also, ask yourself: If you make the wrong choice, what's the worst that can happen? You might arrive at
Disneyland Paris 10 minutes later. What's 10 minutes on a whole day or a whole vacation?
Oh, and one more thing, to make it even easier for tourists, the station 'Marne la Vallee - Chessy' has a Mickey sign next to it.
Relax, it's only France