The first dance I learned in my life, as soon as I could walk, was the Greek form of belly dance called Tsifteteli. Later, when I started with school I learned many Greek Folkdances. At highschool I performed Greek Folkdances with the Folkdance troupe of my school. I also did a lot of rhythmical gymnastic and different other sports in my teenage time. In my early twenties I started with the Arabian Raqs Sharqi and my Tsifteteli skills were of course very helpful. I suppose, I learned the Arabian belly dance quicker than somebody, who hasn't grown up with some form of belly dance. Later I did martial arts for about 10 years, Ballroom and latin dances for about 2 years and I've been dancing Salsa, Merengue and Bachate for more than 10 years. Since belly dance has always been my primary dance I can only confirm the opposite process: belly dance helped me to learn the other dances and body exercises easier
I have many students who have dealt with other dance styles, before they started to learn belly dance. It depends on the person and on how intensive the training of the the other dances was, if it makes it easier or harder for them to learn belly dance. I have a lady for example who has been dancing Israelian Folk dances and Latin dances for very many years and although she can move very well, she has big difficulties with the belly dance moves. But there are also people, who learn quicker because of a former dance training.