Both merchant and affiliate can choose his own system language directly on the login screen to his merchant/affiliate panel.
Available languages they can choose from depends on the currently imported and active languages in the Post Affiliate Pro software. By default there is only one imported language and that is English. If there is only one active language in your Post Affiliate Pro installation the menu to choose the system language is hidden from the login screen.
Merchant can import and even create own custom languages in our best affiliate software. Importing and creating new languages is explained in details in this guide.
Here is a list of all ready to use translations for Post Affiliate Pro software: