How do students' development and experiences affect learning?

How can I tell if a student's language difficulties are because English is his second language or because he has a language disability? I chose this question because I went through the same process that many children face in this current educational system: learning a second language. I know it is hard and it is not something that it can be learned in one year, or two, or three. It is a constant process of acquiring new words,etc. I have felt how people have though that I am stupid or slow because I can't speak sometimes. Many kids feel the same way and I love this article because it focuses in sensitiveness and how teachers should be sensitive about this issue. http://serge.ccsso.org/question_6_4.html