In the Android world, translations are usually found in string resources. See [1], for example.
AFAIK, if you want improved translations, 1) you'd need to either fork AOSP, build a custom ROM, and your users would need to deploy that custom ROM on their devices. or 2) convince Google to accept your changes [2].
Edit: There may be other hacky approaches I'm not aware of. You might find something useful if you trawl the XDA forums [3].