In general Paris is a safe city to travel. The most prevalent crime by far is petty theft. Pickpockets and lots of scams, also be aware with you luggage and purse.
If you need help, the police can be reached by phone by dialing 17 but most of the police do not speak English.
Around tourist areas won't be a problem to find people that do speak English.
From the Charles de Gaulle Airport to downtown area which passes through the poor suburbs of Seine-Saint Denis. Be also aware that thieves may start a fights to steal your valuables.
Trains have cameras every where and thieves are much less likely to use them.