Now you can use Google Translate offline if you have an Android device. Just install the latest version of the Google Translate for Android and you can download the core translation files for more than 50 languages. If you've already installed the application, you need to manually update it. The compressed language files have about 150MB (200MB after extracting the files from the archive) and are stored on your SD card if your Android device has one.
"You can select [Offline Languages] in the app menu to see all the offline language packages available for download. To enable offline translation between any two languages, you just need to select them in the offline languages menu. Once the packages are downloaded, you're good to go," informs Google.
The application works well offline, but you may need to wait a few seconds if you want to translate long texts. Offline translations are less accurate since Google lets you download some simplified language models.
Here's a side-by-side comparison of the offline and online translations of a French text from Le Monde: