Actually, I'm a huge fan of Translucent, and have been keeping the Wiki updated:
http://en.wikipedia.org/wiki/Translucent_%28manga%29
They mentioned to me in email that they're going to finish translating it officially (through Dark Horse), they just donno when.
But yeah, her invisibility is a disease, called "Translucent Syndrome." It's emotions based, sorta -- about one week in four she's at least partially invisible, and being happy makes it last less and affect her less, being sad or stressed makes it last longer and be more severe.
Typically she's about 80% translucent over 100% of her body. However, there are complications:
* If she spends more than 2 weeks translucent, she'll never recover
* Anyone with Translucent Syndrome has a chance of gaining "Fully Transparent Syndrome" which is much more severe -- 99-100% invisibility all the time. One of the characters in the manga goes from Translucent to Transparent really quickly, and doesn't recover.
* Yes, Shizuka gains "Fully Transparent Syndrome" in volume 4, which is bad -- no one who spends more than a week invisible recovers.
* The impression I get from Volume 5 is that she's NOT supposed to have Translucent syndrome anymore -- that after recovering from Fully Transparent Syndrome she's supposed to be cured. Given that she starts going translucent again, and upon becoming translucent, she's never shown in the manga "normal" again, I donno...
Volume 3 was released last Monday to stores, and I got my copy in yesterday from TFAW.com. It's clean but I LOOOOVE it.