If you cannot translate while you are offline, follow the troubleshooting steps below:
Let’s first check if the language has been downloaded for offline use. You can check this by tapping the 3 dots at the top 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.