The users need not be logged in to change the language of the portal. Please navigate to the customer portal as a user to see the dropdown of your languages supported in your helpdesk. When you select from this list, the portal is available in that chosen language.
As of now, there is no feature to restrict this to only logged in users. You could always keep the language hidden by not clicking the eye icon (which makes it visible on the portal) next to it in the Admin -> Account -> Helpdesk Settings -> Manage Languages.