Hi Hannes,
The messages are usually maintained in a message class. E.g., for the message class FDT_TRANSPORT you can translate the messages, e.g., via SE91->Goto->Translation.
I am not sure what exactly your second question is about. Are you talking about the BRFplus workbench, or a custom program and UI? Could you explain more in details your question, please?
Regards,
Shahram