Here's why:
* "My aunt speaks three languages fluently." This is a complete sentence with a subject (aunt) and a verb (speaks).
* "She is a translator at the UN." This is also a complete sentence with a subject (she) and a verb (is).
The two sentences are joined with a comma, which is not correct because it doesn't have a conjunction (like "and" or "but") to connect them properly.
To fix it, you could:
* Use a conjunction: "My aunt speaks three languages fluently, and she is a translator at the UN."
* Use a semicolon: "My aunt speaks three languages fluently; she is a translator at the UN."
* Separate them into two sentences: "My aunt speaks three languages fluently. She is a translator at the UN."