You will need to have an internet connection in order to log in to the Text United app and open a translation package. After opening it you can switch offline and work in the app without an internet connection.
From the menu on the top select the Switch offline option.
You will be able to continue with translation changes will be saved to your local hard disk. The translation will be transferred to the server as soon as you switch back online (using the same button) or next time you open the Text United app.