Can't say I really agree with Langebaan getting super crowded- not with kiters anyway. I've never had a problem with there being too many kiters. Sure, you'll get the learners starting outside of Pearlys and going downwind, but as long as you give them room, there's heaps of space!
I always used to find if you launched and took off from front of Pearlys and took a bit of time getting upwind and go back South (left), further into the lagoon, you usually have that area all to yourself! You certainly won't get any learners there anyway coz they can't get there. I always fancied going around the island too but never had the balls to do so in case there was some wind shadow the other side.
Peak time of year yes, it is very busy with holiday makers. We stayed with the In-Laws the other year and I went for a kite just after Xmas/ NY. It was hectic- trying to set-up/ launch through the throngs. It was in my view very sketchy with kids n folk all over the place. Managed to get a break and threw the think up quick and on the water.
As stated previously, just avoid going mid December to around mid/ late Jan and you should be fine once the schools and associated rabble go back.
Down the road in Table Bay, you got so many miles of kite beaches, you wont have to worry about crowds.
Nice webcam here outside the Best shop on Blouberg' front. http://www.bestkiteboardingafrica.com/Webcam.php
and a few others....http://www.xplorer.co.za/local/webcams/index.html
You see that "Table-cloth", appearing over the mountain, you know the Dr is blowing