Regarding the travel prices. Again i wouldn't be 100% sure on price since i live in Europe, fares etc may vary but iv found that an adult single way ticket usually costs around close to 1000 euros. Although if youth rates apply you may get it for alot cheaper. Iv also found return flights for any age seem to be cheaper than single way.

Regarding the knowing Japanese and getting around thing, if you plan on staying there for a few weeks and sticking mainly to the big cities or tourist spots you will be fine without knowing alot of the language. Having a phrasebook handy will be useful for some things you may have to ask like simply " Ikura desu ka? " which means 'how much is it?' while pointing at somthing at a menu or whatever having a travel guide as i said, for example the Lonely Planet travel guide (who do loads of travel guides for countries), they have lots of useful information and maps on where to go etc

A Youth hostel is simply accomodation where alot of the facilities you share with the rest of the building, be it sleeping room, kitchens, bathrooms etc. Everything in a youth hostel is communal and the cheapest form of accomodation would be sleeping in a dorm with others. You can have your own private room in the hostels but cost a bit more.

Remember, there is a difference between HOTELS and HOSTELS . But yes, staying in a Hotel instead of a Hostel will work out more expensive.

Feel free if you need to ask anymore questions