I agree, the accent is not a fabrication but truly created through the voice actor. However, it is clear that the tone of the voice is altered in the game to reflect Tali having to wear a mask. Listen here starting at about 3:05. Sorry for going off topic though.
"
Ash- I was born in New York City. Both of my parents are actors and they tried to steer me away from the business! They told me that if there’s anything else I want to do I should do it. It was kind of hopeless though, I loved acting. I did my first professional play when I was 8 years old in New York. It was a play based in New Zealand, Ron’s Garden by Julie McKee. I had to learn a pretty hardcore Kiwi accent. That was the beginning of my exposure to dialects, which eventually led to Tali’s unidentifiable pseudo Eastern European, Quarian Gypsy accent in Mass Effect."
SourceIt's a fabrication.