Not all languages use "Re" word for replies.
This is used in English language, but, for example, in Romanian is "Ră" from "Răspuns" or.. another part/short of this word.
I think it must included this in language files and must be translated in each language.

This shortened word can be modified in posting.php:
<!-------------- find start -------------->
$post_data['post_subject'] = ((strpos($post_data['post_subject'], 'Re: ') !== 0) ? 'Re: ' : '') . censor_text($post_data['post_subject']);
<!-------------- find end -------------->
<!-------------- replace start -------------->
Re:
<!-------------- replace end -------------->
with what you want.
Replace both of "Re:".