I assume you're looking to get the i9295 (international) and not the i537 (AT&T), so I'll try to answer these:
I want to buy this phone and have some questions
1 - does it have multiwindow like 9500?
It does have multi-window, the firmware and interface is almost exactly like the i9500.
2 - does it have modes of camera like 9500 for exampls drama shot , slow and fast motion?
The camera has several modes including Aqua Mode where you can take a picture by pressing the volume button. However it does not have slow or fast motion. The camera on the i9295 is different than the one on the i9500. The S4 Active has an 8MP camera while the regular S4 has a 13MP camera. People have asked about porting the slow/fast motion to this phone, but so far nothing.
3 - does it have sensors for air view , smart pause and etc...
It has all of these features. Again the firmware is nearly identical to the i9500.
4 - does it have voice command?for answer call , capture image amd .....
Yes, see above.
5 - does it have 4.3 official rom?
The i537 has an official 4.3 ROM, but the i9295 does not yet. More on this is explained in the next question.
6 - why developers are not very active for thia phone like 9500 and 9505?
Part of the problem is the way Samsung released this phone. They released it about 6 months after they released the regular S4, so everybody who wanted a new phone got the S4 and when the S4 Active came out there were not enough changes and it wasn't marketed in a way that people were willing to buy a new phone so quickly. Therefore, the S4 Active didn't sell nearly the numbers that the S4 did. Secondly, they marketed it as a waterproof phone and if you look through the forums you'll see that it's not waterproof, but water-resistent. A lot of people ended up destroying their phones by doing exactly what Samsung did in the commercials. Samsung has offered a 1-time replacement due to water damage, but all of these claims caused them to stop marketing the phone. It seems that Samsung has really just "forgotten" about this phone since it has a small userbase. I think they're just hoping it will go away with all the problems they've had.
Another reason is the i537 (AT&T) has a locked bootloader so development has really been stalled, really on all AT&T and Verizon Samsung phones that have a locked bootloader. That said there are ways you can use ROMs developed for the regular S4 on the S4 Active. So although there's not much development in the S4 Active forums you can use ROMs meant for the regular S4 so there is actually a lot of choice.
7 - does it have "increase touch sensitivity?
Settings->My Device->Display->High Touch Sensitivity
I really like this phone, particularly the hard keys at the bottom. I too wish there was more development but unfortunately it just didn't sell that well. I haven't had any problems with it and I think the LCD screen looks much better than the Amoled on the regular S4, particularly in direct sunlight. Hopefully this helps.