Google Translate has an app, you can copy-paste in it if you wish. You
can download GT translation files for Chinese once and use it offline.
Not sure this will work for the voice/OCR input part as well.
Moreover, "Yandex Translate" app (not from Google but quite good) has
a widget for translating web pages in-place (it's in "share" popup of
Safari browser).
As an aside - Google have acquired a licence to Tencent's translation
engine a couple of years ago, which massively improved the quality of
GT output. Which makes me sure there are Chinese apps that do
translation even better.
PS an opinion on Pleco - it is an excellent offline dictionary lookup
shell, but it lacks in other areas. Use the best tool for your
particular task :)
Regards,
Vitaly