By default Rich Returns will detect the customer language based on the browser language set by the user and display the customer portal accordingly in different languages. This approach is the most widely used in eCommerce (e.g. by Amazon) and most user friendly since it really respects the settings of the user.
Find the pre-translated languages in the Rich Returns' Dashboard under Translations / Languages
These are enabled by default, no action needed from your side
Test it for yourself, e.g. in the Firefox browser
Settings / General / Language
Add a new language at the bottom or move your language to the top
Reload the returns portal page of yours to see the change reflected
You can customize most texts on the returns portal, see Customizing the language / text on all pages
Navigate to Translations / Customize / Select your language
Click "Activate this language on the returns portal"
Click "Make this the default language"
This language will be shown by default when no other custom language is enabled. If you have enabled only this one custom language, then it will always be shown.