Here's a case where we actually know the exact origins of the title. Lahiri chose the title because of a friend of hers who worked as an interpreter in a doctor's office in Brookline, Massachusetts. (Go ahead and check the story out yourself.)
But what's the larger significance of the title? We'll direct you to the title story, in which Mr. Kapasi talks to Mrs. Das about his second job as an interpreter for a doctor. Mrs. Das thinks Mr. Kapasi's job as an interpreter is "romantic" (Interpreter of Maladies 61), "a big responsibility" (IM 75).
Think of those two concepts she names—romance and responsibility—as threads that run through the entire collection. Every story in Interpreter of Maladies weighs the risks of pursuing one's romantic nature against the demands of reality. Which is the heavier burden? Which leads to a more fulfilling life—the romantic dream or the responsibilities of everyday life?
Now consider the function of the interpreter: what kind of responsibility does an interpreter have to his or her audience? Isn't all storytelling a kind of interpretation, freighted with the same risks? And aren't all stories stories about "maladies," whether real or imagined? You could say we're all sufferers from the malady of figuring out how to connect with the people around us. And we think Lahiri is an amazing interpreter.