An update to the Google Translate app for Android is going out today, adding support for offline translations for fifty languages. It's a big upgrade for Google Translate, which until now has required an internet connection. (We all know how expensive international roaming can be.)
The new version supports Android 2.3 Gingerbread and above, and languages can be downloaded from the new "Offline languages" menu. A word of warning -- downloading an entire language to your phone takes just as much space as you'd imagine. With only English and German installed, we'd already taken up a hefty 280MB of internal storage.
There are also some limitations to offline mode -- it's currently text-only, so voice translation and image translation don't work offline.
If you've already got Google Translate installed, hit the Play Store to get the latest update. If not, you can use the handy Google Play link above.