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/1883668-how-to-download-languages-for-offline-use
and here: https://microsofttranslator.uservoice.com/knowledgebase/articles/1883671-how-to-use-microsoft-translator-when-you-are-offli