When you speak Thai in the slums or anywhere else it opens doors for you. I do not know how many times I have had people seem stand offish or shy or angry but after I speak Thai, make a joke, ask them about themselves or tell them about myself, the photos flow. Not sure how photographers can make good photographs in places where they cannot speak the language. I find that language, talking to those I photograph is absolutely crucial to my image making. It also helps me understand the people I am with which hopefully leads to more truth in the images.
Plus its fun to speak Thai!