Go to Menu and search for Offline languages option.
Then tap on the pin icon adjacent to the languages which you like to download.
At this time, you will notice to choose optional internet connection like wifi or 3G mobile internet to download the required language pack over the internet.
Once the language packs started to be downloaded, you will notice their download steps under the Installed section.
Use Google Translate offline on Android
Switch your Android device in Airplane mode to disable all the internet connection.
Next launch Google Translate app.
When you use Google Translate app during offline mode, you will find “OFFLINE” mark at the bottom of translated text section.
Might be this information you can found also another website or webpage but in a serious mode just here for your help of guide otherwise there is always open 24 hour working google customer support professionals who make your problems easy to understand.