Change email reply's automatic object

I very much doubt this is editable anywhere - because it doesn't actually mean what people think it means…

Everybody thinks re means reply or regarding, but actually it is a word in & of itself.

Re is the ablative form of the latin word res, meaning 'thing' or 'affair', or 'in the manner of'.
It is not an abbreviation at all.

Refs:
^{Which is an abbreviation, of 'references' ;-)}
https://www.dailywritingtips.com/regarding-re/
https://thewritecorner.wordpress.com/2010/03/20/what-does-re-stand-for/
https://english.stackexchange.com/a/2520

Another reason to not ever change it is that it forms part of the email standard RFC 822 - Internet Message Format, which specifies it as an accepted method to start a title - no email app will ever be looking for anything other than re when it threads successive emails with the same title.

When used in a reply, the field body MAY start with the string "Re: " (from the Latin "res", in the matter of) followed by the contents of the "Subject:" field body of the original message.

BTW, I knew where to start researching this information because the first job I had fresh out of school involved legal writing, which frequently uses Re: to indicate subject matter & not to indicate it is a reply.