Google Translate just added a very useful feature to Android. The 'Tap to Translate' feature allows you to use the translation text from within any application.
For example, when you copy a text from WhatsApp, the Translation bubble will appear. So you can click there to see the translated text.
From the same point you can also change the languages or make a new translation.
Meanwhile, iOS users will have Android's Offline mode. THE mode χωρίς σύνδεση (Offline mode) θα κάνει μικρότερες μεταφράσεις χωρίς να χρειάζεται σύνδεση στο διαnetwork or mobile data connections such as Google Translate.
You should download language packs that are around 25MB each.
As Google says, packet size has dropped by one 90%. You can enable offline languages by pressing the arrow next to the language name.