Can JCo send multi-language text to SAP? Do I have to configure something in SAPGUI to make this happen?
I am not sure that I understand this question properly, so let me know if I did not answer the question. JCo itself does not know about languages because it is just a layer around RFC which also does not care about languages. The language used by an application function depends on the language you specified for the connection. If an application function allows you to specify a language code (some do) then you can select a language at runtime.