If you want to make a language version of a course using an XLIFF file, start by duplicating the original course and continue your work with the XLIFF file. The button to duplicate the course can be found in the Composer home screen of each course.
Open the new course you are duplicating, then click the "Export and Publish" button in the top right-hand corner, which is the left-most button in a row of five buttons. Now you will see a button called XLIFF, among other things, and you can download the texts of your course in XLIFF format.
Once the texts have been translated and you want to import the translated XLIFF back, reopen the manuscript you created from the original course. Then click on the "translate" button at the top right. The button is located right next to the "Export and Publish" button.
On the form that appears, first select the target language. A checkbox will specify whether the XLIFF file has been translated by replacing the original texts or whether a new "target" language has been created alongside.
Then select the XLIFF file from your device by clicking on the radio button. Finally, click the "translate" button and you will see a confirmation that the translation was successful.