Have you ever been amazed at how well some people manage to express themselves in a foreign language?
Such a good standard cannot be reached overnight, and competent speakers have normally spent many years in contact with the language and culture in question.
You may also have noticed that an individual’s level of competence is not necessarily proportional to the number of years the person actually spent in contact with the language. There are many other factors at play, such as our reasons for learning the language, the kinds of strategies that we use when interacting in the target language, or the particular situations in which we normally use it. Less obvious factors, such as an unconscious willingness to retain our 'foreign' identity, can make otherwise perfectly fluent speakers hold on to their distinctive foreign accent.
In many ways, the way we speak a language tells the story of our relationship with it.