The state police and the federales often set up checkpoints on highways. They are looking for drug traffickers, but they will occasionally ask to see your tourist permits or vehicle import permits. They are usually efficient and quick, and chances are good that if you look like tourists, you'll simply be waved on.
talk
Spanish is the official language. English is widely taught as a second language. People will speak it in various levels, but within the Metropolitan area of Monterrey is were most likely fluent speakers will be found.