She obviously speaks English with a Slovenian acescent, but that doesn’t mean she isn’t fluent, or don’t have a wide vocabulary in English.
“Everybody” has some sort of accent when speaking English, incl. native speakers. “American English” has many accents, as does “British English”, or any other of the many "native English speakers around the world.
How many words are needed to claim “fluency” can be debated:
How many words do you need to know to be fluent in English?.
PS> Here is an article that take up the question of how many languages Melania Trump actually speak: