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>