Master is a telic verb: it signifies the change of state from non-mastery (or incomplete mastery) to mastery, in the same way that learn signifies the change of state from ignorance to knowledge.
The simple present form with telic verbs ordinarily signifies habitual, repeated action. If we say of someone that He builds houses, we mean that he is a builder by profession: he spends every day working at building one house or another.
But ordinarily, master doesn't work like this: when you master a skill you enter into a more or less permanent state of being able to perform that skill. You don't master a skill over and over again. So Susan masters (or doesn't master) Japanese doesn't fit very well with the meaning of the verb.
And the present perfect doesn't signify that "something happened in the past and [is] still happening in the present"; it signifies that the past event establishes a present state. With master, specifically, if you say of someone that they have mastered a skill, that means that at sometime in the past she passed from a state of non-mastery to a state of mastery which continues into the present. Another way of expressing that state of mastery is to say that she is a master of (the skill).
By the same token, if you say that someone has not mastered a skill, that means that she is still in a state of non-mastery, or incomplete mastery. That is the case with Susan:
She is not yet a master of Japanese, but she can communicate.