The JSTL (JavaServer Pages Standard Tag Library) fmt tag library provides a set of tags for formatting and internationalizing data in JSP (JavaServer Pages) pages. One of the tags in the fmt tag library is the <fmt:setLocale> tag, which is used to set the locale to use for formatting and localization.

The <fmt:setLocale> tag is used to set the locale to use for formatting and localization. The locale determines how data such as numbers, dates, and messages are formatted, and also which localized versions of messages and other data are used. The tag should be placed at the top of the JSP page, before any data is sent to the server. The syntax of the tag is as follows:

<fmt:setLocale value="localeValue" [scope="page|request|session|application"] />

where localeValue is the value of the locale to use, and scope is the scope of the attribute that stores the locale (optional).

For example, to set the locale to use for formatting and localization to US English, you would use the following tag:

<fmt:setLocale value="en_US" />

This tag sets the locale to use for formatting and localization to US English.

You can also specify the scope of the attribute that stores the locale. If no scope is specified, the attribute is stored in the page scope. For example, to store the locale in the session scope, you would use the following tag:

<fmt:setLocale value="en_US" scope="session" />

This tag sets the locale to use for formatting and localization to US English and stores it in the session scope.

Note that the fmt tag library must be imported at the beginning of the JSP page using the following tag:

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

This imports the fmt tag library and assigns it a prefix of fmt.