<fmt:setLocale locale='en' scope='session'/> should store an English locale in the javax.servlet.jsp.jstl.fmt.locale config setting for session scope. Instead, I get this validation error: Validation error messages from tag library fmt null: Illegal scope attribute without var in "fmt:setLocale" tag. (From 20020422 build)
Taglib directive of the page that triggered this issue was using a custom URI ("standard-fmt") instead of the "standard" URI ("http://java.sun.com/jstl/fmt") that is specified in fmt.tld. Because of this, the FMT TLV did not consider the page's tags as JSTL tags, prompting it to flag a dangling-scope error where one was not expected.