Locale part contains invalid characters

Im Logfile gibt es immer wieder Einträge der Form

[http-nio-8080-exec-324] ERROR o.a.c.c.C.[.[.[.[dispatcherServlet] - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalArgumentException: Locale part "en'" contains invalid characters] with root cause
 java.lang.IllegalArgumentException: Locale part "en'" contains invalid characters
   at org.springframework.util.StringUtils.validateLocalePart(StringUtils.java:902)
   at org.springframework.util.StringUtils.parseLocale(StringUtils.java:839)
   at org.springframework.web.servlet.i18n.LocaleChangeInterceptor.parseLocaleValue(LocaleChangeInterceptor.java:195)
   at org.springframework.web.servlet.i18n.LocaleChangeInterceptor.preHandle(LocaleChangeInterceptor.java:154)

Reproduzieren lässt sich der Fehler:

curl http://localhost:8080?lang=en/