I am most definitely writing about that in my PS, as that is exactly what I want to do. But, as others have said, I would recommend having experiences that back it up. I have worked in various medical and non-medical settings with hispanics, and know for sure that I want to work somewhere where I will be helping this population. I was a Spanish major, speak fluent Spanish (I am not hispanic at all though), I have volunteered for the past year as a translator at free clinics, and I taught theatre for 2 years to (not so) bilingual students. I think these experiences will definitely demonstrate to adcoms that I am not just writing it in there to have a better chance of acceptance. I would be cautious though, if you don't have experiences like these, as anyone can say they want to work with the underserved, but many don't end up really doing it.
edit: just saw you posted while I was writing my response. I would suggest that you focus on your experiences within the US more than your abroad experiences, as right now you have much more abroad than you do within the US. I think med schools would be wary if they thought you were going to get trained here, and then take off forever to a 3rd world country. I would definitely mention all of it, and talk maybe about how you realized you want to work with the underserved while you were wherever you were, and that made you want to help the same population within the US or something. Just be careful how you word it.