The system language on your PPM instance is used for the web service operations that write data into PPM (create and update services). To work with the data in a supported non languages supported (and enabled) on PPM, specify the session language in the SOAP header.
PPM does not support working with the data in multiple languages in a single web service call. Multiple, separate web service transactions are required, and you must specify the session language for each.
Web service special commands also rely on the selected session language. To work with the data in a different language, you must change your session language.
For information about the Web services available for you to use in your PPM deployment, see the Web Services Guide.