English is widely spoken in the tourist areas so you'll be sound. What are you planning on doing, exploring or working? You can have a maximum 180 day visa in Mexico to explore but you can't work legally.
Cancun is a city with over 300,000 people so same we any city theres its social problems but from what I remember no worse than anywhere else. The drug problems are not so prevalent there as in the violence and that so as long as you're not a tit and have your normal wits about you then you'll be sound.
Boss part of the world though. Explore and enjoy. Or use it as a starting point and head down to Belize straight down the 307 highway and do Central America lid.