When replying to a message no additional "Re:" label should be inserted before the subject if such a translated label is found. The client therefore should know the commonly used / translated versions of "Re:" preferably editable via preferences.
i.e. In Dutch Betr: is used and in ...
I followed the advice of David (see the linked idea) and created this idea.
In the Slavic group of languages for the name of the person using a 3 ID: Surname (family name), First name and Patronymic.
First name - for centuries (and some thousands of ...