Google Translate just added a very useful one feature on Android. The 'Tap to Translate' feature allows you to use text translation from within any app.
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 διαδίκτυο ή συνδέσεις δεδομένων κινητής τηλεφωνίας όπως το Google Translate.
You should download language packs that are around 25MB each.
As Google reports, packet size has been reduced by 90%. You can enable offline languages by tapping it arrow next to the language name.