Take the following steps to translate a text when you are offline: 

• Open Microsoft Translator
• Tap the Keyboard icon on the main screen
• Choose the from-language by tapping on the language at the bottom left of your screen
• Tap on the language you would like to translate from
• Choose the to-language by tapping on the language at the bottom right of your screen 
• Tap on the language you would like to translate to
• Tap Type to translate at the top of your screen and a keyboard appears
• Type the sentence you would like to translate
• Tap Done 
• The translation appears in the middle of you screen

Offline language packs work in Text and Camera Translation mode. They will not work in Voice or Conversation mode.