The official language is French, and while many Parisians speak English, they appreciate when visitors make an effort to speak a little French. In the tourist districts, many people, especially younger generations and those in the service industry speak decent English.
Outside of Paris, in smaller towns or less touristy areas, English is less common. Basic French can be quite useful here for communication, as locals may not be as proficient in English.
Public transportation in Paris (metro, buses) has bilingual signs in French and English, and announcements on major train lines are often in both languages. Staff at major stations and airports usually speak English.
In Paris, you'll generally get by with English, but we recommend learning a few key phrases in French, such as "Bonjour" (Hello), "Merci" (Thank you), and "Excusez-moi" (Excuse me). They can go a long way while interacting with locals.