I graduated June 2003 and will finish 5th Pathway this June. I matched in Ob/Gyn.
First, the first 2 years are 60% in English. You start with some classes in Spanish from the first semester (especially the clinical skills class, called IPM) and will have to go to clinics each semester in the areas surrounding UAG, so Spanish is needed there, too. You need to be relatively fluent by 5th semester when you go to the hospital because all the classes and tests are in Spanish. They will let you use a regular English/Spanish dictionary (i.e., not a medical one) for tests, though. Many of the professors understand English, although they may not be comfortable enough to speak it or they just let you think they don't understand it (so watch out -- I know of people who said things thinking the teacher wouldn't understand...).
Don't be too disappointed when you visit the campus. The main campus is nice, but ICB, the Basic Sciences campus, leaves a lot to be desired. The library looks like it belongs at an elementary school... UAG takes a lot of self-study. Also, you need to start studying for the Boards on Day One, or the 4th semester Kaplan review course will not be a review, and you will not pass. There are people from my class who still have not passed Step 1 (and remember I said I graduated last June), which means they haven't started 5PW yet. UAG changed the rules and now you must pass Step 1 before you are allowed to progress to the third year but it wasn't like that for us. When you get to HAL, the Hospital Angel Lea?o, where we do the third year (and the fourth if you don't go to the US for it) is a little better than ICB -- the library is much better, but you shouldn't go there expecting to see the equivalent of UCLA, as I did.
Now, I say all this as someone who really enjoyed her experiences in Mexico, graduated at the top of my class, did Coop during the fourth year, got my desired hospital for 5PW, and am going to my top choice for residency in the field I chose. This is because I worked hard, and didn't party. It's easy to fall into the trap of procrastinating and going to the beach because there are always old tests to study from and you can pass the classes that way. Let me tell you something: of the people I know who passed Step 1 and graduated and got into 5PW, most of us did not depend on the old tests, but referred to them at some point during our independent studying. You'll see what I mean when you get there.
You should search the Forums/Archives for posts relating to UAG and/or 5PW and you'll get a lot of info. If you have any specific questions that aren't covered there, feel free to PM me.