When you open the app and select a language, you will see an arrow pointing down to line.
Just press on the icon and it will download.
Once you have downloaded the language for offline use it goes to the devices memory and it is accessible when your are using the app AND you are not connected to the internet.
Please make sure that there is a check mark next to the language in the select languages menu. This means that the language is downloaded and that you are able to use it offline.
Note: When translating while offline do not use (Detect) as your 'from-language' but choose one of the languages you have downloaded for off-line use.
Offline language packs work in Text and Camera Translation. They will not work in Voice or Conversation.