In the shorter term Japanese is showing the best signs of good sales but longer term China with it's massive population looks like the one to watch...The level of political control exercised by China over the internet is an uncertainty thats difficult to measure when investing however.
Arabic is a good one and interest is increasing in this area. One advantage here that .com and .net have is that the Arabic speaking world is spread over a large number of different countries so country tld's don't look like they will be as popular in IDN terms here.
Don't forget Russia as some of the same Cyrillic script is also used in places like Ukraine and Bulgaria too and then there is the big Indian question....Will they use English .in and .co.in or once IDN's in local languages (Hindi, Tamil, Telegu etc) become known will they start to use those instead ? India will be massive as again population is huge but slightly different to China in that many different languages are used there which could make investing tricky as it's hard to tell which horse to back at the moment !
We shouldn't overlook the sales that have already happened in latin IDN too...German in particular but French are doing well also and Spanish, Portuguese (which both have speaking populations in South America too) and smaller languages like Greek are also being speculated at the present time....
I also have IDN's in other languages that I haven't even mentioned so there is a whole world of opportunity out there if you are prepared to think outside the box and check out the next big growth area....
All of the above is just my two pence of course...
David