I use English to write in keynote and need to convert the whole thing to Korean without having to re-type the whole keynote.
Apple has never included language translation in any apps. You will have to use a human or machine translator to replace the existing text. The best machine service is
I think they also have an iPad app.
When using machine translation, it's really important to either know the language yourself of have someone who does know it check the results, because it is not difficult for damaging/embarassing errors to be made.