Google updated its Google Translate app for Android today to version 2.6. The new version, available for Android 2.3 and up, supports the translation of vertical text in Korean, Japanese, and Chinese with your camera. The big news, however, 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.
When the language packs begin downloading, you'll see their progress under the Installed section. To test the offline language packs, put your phone in Airplane mode. When using Google Translate in offline mode, you'll see "OFFLINE" underneath the translated text.
Though the offline feature is a welcome addition to Google Translate, keep in mind that it doesn't have all the features of the online version. For example, there's no option to have the translated text read aloud. Also, the iOS version does not yet support offline language packs and there's no word from Google on when or if it will.