Languages are different, and use sounds that are different from each other. But people who learn a new language often retain an accent from their original language. How does that happen? Folks tend to retain bits and pieces of their original language when operating in a new one. This happens at such an elemental level that we don't even realize it. You don't see it in people who learned two or more languages in their early childhood, because they are both his "native language."
Linguist Arika Okrent gives us examples that illustrate this concept, while an artist shows us what's happening on a whiteboard. -via Mental Floss