Hm, changing the code in inc/parser/xhtml.php is what I did (without seeing this FR), but darcs had problems to find the right places for subsequent patches and put ^^^^ and so on into the code. That's why I do not like code changes.
At the moment, I capture (in ACH template's main.php) the call of tpl_content() and do a str_replace for the quotation marks.
Since it is not really a pain to change the code from a constant to a variable, I thought this was possible to do right in the repository. Whether the variables are $lang['something'] or $conf['something'] doesn't really matter. But obviously it is too hard to do, since one year after this call was openend I created a new one (FS#881
) - I searched for "quote", but did not find this feature request.