According to section 8.10.3 of the jstl spec 'javax.servlet.jsp.jstl.fmt.basename' could be used to specify the default bundle that <fmt:message> tags should use. Specifying this parameter in web.xml however causes a <fmt:message> tag to display "???<key>???". Putting a <fmt:setBundle basename="<xxx>"/> at the beginning of the jsp containing the <fmt:message> tag does work as it should.
I believe you are not looking at the final release of the JSTL spec. The configuration setting is defined as follows in section 8.11.3: 8.11.3 I18n Localization Context Specifies the default i18n localization context to be used by the i18n-capable formatting actions. A String value is interpreted as a resource bundle basename. Variable name: javax.servlet.jsp.jstl.fmt.localizationContext Java Constant: Config.FMT_LOCALIZATION_CONTEXT Type: String or javax.servlet.jsp.jstl.fmt.LocalizationContext Set by: <fmt:setBundle> Used by: <fmt:message>, <fmt:formatNumber>, <fmt:parseNumber>, <fmt:formatDate>, <fmt:parseDate>