Let's first check if the language has been downloaded for offline use.
You can check this by tapping the gear lever at the bottom right of your screen
and then by tapping "Offline languages". If the language you wanted
to download is at the top of the list with a checkmark next to it, it means
that the language is downloaded.
Secondly, please know that
you'll need to download both the from-language and the to-language packs for
offline use. The English language pack has already been downloaded. Offline
language packs work in Text and Camera Translations. They will not work in
Voice or Conversation mode.
If both your languages are
downloaded and you still cannot translate while offline, then change your phone
setting to airplane mode so that the app 'knows' you are offline.