Probably something like:
 
<script type="text/javascript">
     messages = {<% { java.util.Locale locale = (java.util.Locale)session.getAttribute(org.apache.struts.Globals.LOCALE_KEY);
     java.text.DecimalFormatSymbols df = new java.text.DecimalFormatSymbols(locale); %>
     "locale":"<%=locale.getLanguage()%>",
     "decsep":"<%=df.getDecimalSeparator()%>",
     "groupsep":"<%=df.getGroupingSeparator()%>",
     "decimals":"<%=java.text.NumberFormat.getNumberInstance(locale).getMaximumFractionDigits()%>"}
</script>