I did the following test:
I put the phone into "Airplane" mode (this should turn off Wi-Fi and cellular data), ran the example program. It correctly translated English.
I have seen mentioned in a few places that in order to translate off-line you need to have "Language Packs" for that language installed. These packs are downloaded from Google.
I have never verified this – it would be an interesting test. I think you download Language Packs in Settings – Language and Input.