To be able to use Translator while you are offline you need to first download the language packs. Do this while you are on a Wi-Fi connection and your device is not in battery saving mode. The language has been downloaded when you see that the language has moved to the top of the list and has a checkmark next to it.
Offline language packs work in Text and Camera Translations. They will not work in Voice or Conversation mode.
You can find the steps on how to download languages and how to use the app while offline here: https://microsofttranslator.uservoice.com/knowledgebase/articles/1883569-how-to-download-languages-for-offline-use
and here: https://microsofttranslator.uservoice.com/knowledgebase/articles/1883572-how-to-use-microsoft-translator-when-you-are-offli