Because Tulum is such a popular international tourist destination, English is generally understood there. However, you're in Mexico, a Spanish-speaking country! Not everyone you encounter will speak English, so it's a good idea (and respectful of the people who live there) to brush up on your Spanish or learn some basic conversational phrases so that you're able to communicate with locals. If you hit only touristy spots, communication shouldn't be a problem, but many cab drivers and owners of local businesses, shops, and restaurants don't speak as much English and will greatly appreciate your efforts to speak Spanish. Even a simple "gracias" goes a long way.