Google updated its Google Translate app for Android today to version 2.6. The big news is that it finally supports offline use with downloadable language packs.
To download one or more of the 50 available language packs, go to Menu > Offline languages, then tap on the pin icon next to the languages you want to download. You’ll then be prompted to choose whether to download the files over your mobile data network or over Wi-Fi.
The downloadable file package in the case of spanish is around 156.74 MB