Knowing a language well means being able to use it effectively to communicate. This is even more true for a language as peculiar as Japanese.
In fact, knowing the translation of individual words is not enough. You must also know how and when you can use them.
I guess you’ve tried yourself to say something in Japanese only to find that a native speaker would express it in a totally different way. I’m referring here to the different registers the language has.
I’ll just give you one example.