I've done the same thing. If you look at dw-nonfree/htdocs/index.bml , you'll see that I do this:

        $headextra .= <<HEAD;
<script type="text/javascript">
    var ml = new Object();

HEAD
        $headextra .= "    ml.joinheading = \"" . BML::ml( ".create.join_dreamwidth",  { sitename => $LJ::SITENAMESHORT } ) . "\";\n";
        $headextra .= <<HEAD;
    ml.entercode   = "$ML{'.create.enter_code'}";
    ml.usecode     = "$ML{'.create.use_code'}";
    ml.cancel      = "$ML{'.create.cancel'}";

HEAD
        $headextra .= "    var invitelength = " . DW::InviteCodes::CODE_LEN . ";\n";
        $headextra .= "    var siteroot = \"" . $LJ::SITEROOT . "\";\n";
        $headextra .= "</script>\n";

That causes it to make output that looks like this:

<script type="text/javascript">
    var ml = new Object();

    ml.joinheading = "Join YourSite";
    ml.entercode   = "Enter your invite code:";
    ml.usecode     = "Use Code";
    ml.cancel      = "Cancel";

    var invitelength = 20;
    var siteroot = "http://www.daily.hack.dreamwidth.net";
</script>

I can then use ml.joinheading and such in the JavaScript to refer to the strings.