I’m sooo sorry, but I seem too stupid to understand how I can set this (4.), that only the name of my communication partner is shown and not mine in the list. (of 4. Or to show names of other user you have to use fep_message_table_column_content_author
action hook. It needs some custom code. You need participants name then remove own name from them.)
And regarding the language – maybe it is also due to the other language – I was logged into WordPress and followed your link, but I have no idea where to upload the .PO file.
Thank you for your really patient and quick response.