As mentioned above it all depends on what languages QF are looking for at the time as it changes from intake to intake. My feelings are you can never go wrong with an Asian language.
Australian Airlines are recruiting right now for people with advanced levels of all 3 languages you mentioned. Some of QF japanese flights have been allocated to AO so I'm not sure how much of an impact this has had on the Japanese speakers at QF, and if they have more than required at the moment.
I know that QF still fly to Tokyo but do they still fly to other Japanese ports from other cities in Australia? AO are doing KIX, NGO & FUK but only from CNS. Do QF still operate these from elsewhere.