Human brains are wired to soak up languages. Anybody immersed in another language making a minimum effort, will be fluent enough in six months.

People who say they have experienced the contrary typically go to great length to speak and read their native tongue. It doesn't matter how much you tried at home with a book etc... Once you're immersed and have made the decision to understand and speak the local language, it's like your brain goes on steroid. It is difficult to imagine until you've experienced it.