We have occasionally run into and/or met expats from the US and Canada who refuse to intermingle with the locals. This makes absolutely no sense to me at all!!! I think some of the reason for this can be attributed to some expats having an unwillingness to Habla Espanol . Honestly, English and Spanish have alot of vocabulary that overlaps. The main difference is sentence structure and appropriate phrases. Yes, you will have to learn new vocabulary. I won't lie to you. But with some consistent, basic effort you are going to be able to learn to speak basic spanish in about 12 months and become fairly fluent within 2 or 3 years.
I highly recommend that even before you move down you begin to pick up some basics. I have heard that Rosetta Stone is a good program. You have probably seen it on T.V. I would also recommend The Learnables. Go to http://www.learnables.com/ to check out their programs and information. This is the system that I am using with my wife and oldest son. We finished level 1 in about 3 months. On that pace we could finish all four levels in one year. That would put our Spanish proficiency at about an 8th grade level after finishing the program according to their information.
I would also recommend that after you move down that you watch as many movies in English that have Spanish subtitles. Write down some of the common phrases that you see and hear on some of your favorite movies. You would be suprised at how fast you start to pick up the language when you begin to do this. I have met more than one Mexican national who has learned how to speak a fair degree of English just watching American movies like Rocky or regular American T.V. shows. Many of the Mexican nationals who live and work in the tourism industry in Cancun learn to speak English by watching American T.V.
As for daily living, you will want to make bilingual friends as you go along. You will be suprised at how easily that can happen for you. My attorney and co-owner of Yucatan Real Estate Paradise, Francisco Guitterez, is very bilingual. I say "very bilingual" because you meet some people who have some degree of proficiency in English but still have severe limitations. Francisco can carry an ongoing conversation in English and never ask for your help in how to say something. My next door neighbor, landlord's son and son-in-law, the builder that built my home, my son's LOBO Boy Scout leader, ....the list goes on....these guys all speak English even though they are Mexican Nationals.
You will have people that can help you out along the way. Just understand that when you go out you are going to want to converse with the cashiers, waiters, ......whomever. They will know you are a Gringo. Don't worry about it. They will slow down and try to help and talk as simply as they can. Just do your best and you will learn as you go along.
We are here to help those who desire to learn about living, working and retiring in Merida, Mexico. Let us know how we can assist you in your transition. My email is firstname.lastname@example.org. Look forward to hearing from you.