Is it possible to pass in a parameter in the URL and have the language change(Ex: Chinese, French)?
You may be able to use 'lang=' see help link: https://help.payments2us.com/m/userguide/l/1271139-which-parameters-can-i-use-with-the-various-url-parameter-options
If this does not work then you will have to use another Force.com Site, and change the language on the site user record.