I doubt you will find a return flight for as low as 500 Euros in the summer, but you might be lucky. Okinawa is going to add to your trip costs because you will have to fly there out of either Tokyo or Osaka- probably around 20,000 yen at the cheapest for the round trip, or a lot more if you are coming in the high season, i.e. most of August. Ferries are not a lot cheaper.
Keyboards here are always qwerty. There is hiragana on the keys under the alphabet here but everyone uses the alphabet input anyway.
No, electrical outlets are not the same here as in Europe- this is something you can Google actually- try "electrical outlets in japan". Because of this you will need to buy an adaptor. Voltage is also different- 100V compared to 240 in Europe. Laptop, camera and cellphone chargers usually have built-in transformers so should be fine with just an adaptor. My Nintendo DS does not have a built-in transformer so I can't charge it overseas- not sure about the PSP.
For must-sees have a look at a site like
japan-guide.com - Japan Travel and Living Guide - lots of info about travelling in Japan there.
Not many people here speak a lot of English, but tourists who don't speak Japanese visit all the time and get around fine with just simple English and gestures- people who work with tourists usually have some basic English, but don't expect it to be like the Netherlands where everyone's fluent!