When you access the Translations section for the first time, you already have the default language as the first language you can choose to edit. The words that you used as field labels when building your form in the Form Editor are the starting point for all future translations for a particular form, i.e. they are considered as default.
Bellow the default language there is a plus sign and if you click on it a lightbox will pop up where you can select what other languages you want to add to your form and which one to be the default one.
In the same section, you have two buttons: Export to CSV and Import from another form. The former will automatically start downloading a CSV file containing all the translations from your form. The latter is useful if you’ve already set up a multi-language form from where you can import translations.
Important: Copy the tags of the messages that appear on the left in case you want to keep the same structure (e.g. %s is used for links and variables).
You can set up certain conditions in Form Rules that will redirect users to a webpage based on the language they have selected on the form.