Sorry but I have a few more questions now for the fine-tuning ;-):

1. Read Receipt
Here I can’t translate with loco tanslate „Read by“ in „gelesen von“. So I had to change it myself in the php file: pro/includes/class-fep-read-reiceipt.php. But with an update it’s gone again. Is there a way to integrate that into my child theme function file? Unfortunately I’m not that fit in PHP.

2. Instead of username
In fep-header there is e.g. „Welcome: Username“.
Here I wish to show the „first_name last_name“. I use UserPro. Likewise in the communication history. I don’t want to show the username anywhere, but always first and last name. Please look at the screenshot. It will not be possible for all users to enter their first and last names as user names.

3. Hide one setting
In the front-end setting, the user can select three options under the “Settings” button. I would like to hide the first option (“Allow others to send me messages?”), but leave it in the checked setting. How can I do that?

4. Instead of username
In my message box in the front end, my user name is shown to me when I last wrote.
I would like to have the name of my communication partner displayed here by default. Only the picture is unfortunately not enough for me because it quickly becomes confusing. If a message is unread, then I have the red message in addition. How can I do that next to the profile pictures only the name of my communication partner (no username, but first and last name)?

Are you interested in my attempt to translate the .Po file to German? I think I did it quite well, maybe others may adjust it even better in the future if they notice something. I have now done it to the best of my conscience.