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 &amp; 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 &quot;Re: &quot; (from the Latin &quot;res&quot;, in the matter of) followed by
the contents of the &quot;Subject:&quot; 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 &amp; not to indicate it is a reply.