After only 2 weeks in China I can give you some keys to avoid the waste of time I got. See the following apps:
Only one guide for 44 languages, it gives you translation of hundreds useful words and sentence from and to the 44 languages, very well made, easy to use:
Only one subway guide for 33 cities around the world:
700 offline cities maps to save Location and to be geolocalised via GPS:
700 City Maps (this apps is free, but each offline map must be bought, otherwise you just access an online map like google maps)
Currency exchange rates, works offline :
How could we survive before iPhones and iPads ??