The multilingual knowledge base is when you can display your solution articles in the languages configured in your helpdesk. Please navigate to Admin > Account > Helpdesk Settings > where you have an option called 'Manage Languages'.
Please click on this to set up other languages apart from the primary language which would be available on the customer portal after configuration.
Say, for instance, you add French and Spanish to this list and your default language is English, as a customer, they would see the language dropdown at the right end of the navigation bar. Upon choosing the language in the list, the solution articles would display in the chosen language.
This feature is available from the Garden plan. Feel free to reach out to email@example.com in order to upgrade or know more about the feature.