Please navigate to the solutions tab on the global header -> click on the category -> where you could open the folder of articles. When you click on "show languages", you would see the icons of the languages supported in your helpdesk. 

Kindly add the article's translation under each icon in the respective languages. This would make them available in the language chosen in the drop-down on the customer portal. 

You could make an entire folder of articles in one language by following the same instructions.