I can not modify this key inside the language files. The key seems to be wrong, because no matter what I use for the key, the message is always:

"Your reply has been posted successfully."

I tried to change the english and german language file, both had no effects for this one. I think that maybe inside the php file there is not used the key maybe but text? But I have no clue in which file to search for.

The issue is when I send a conversation to someone as successful reminder for the sent message.