Customer portal provides multi-language support to collaboratively work online across organizational, geographical and overcome language barriers.
You need to set message for portal to work with different languages. For setting messages, navigate to Admin -> Developer Tools -> Drop Down Editor.
From drop down editor you will find “customer_portal_app_msgs” tab. From there you can set message for individual languages.
Select language and add messages. It will be reflected on the portal when that language is selected by the portal user.
Note: Default language can be set from CRM view “General Portal Configuration” and if no language is selected it will take from CRM enabled languages.