Unusable on tomcat in some condition

Looks like since 3.4M1 there is a serious issue with tomcat:

• setting the locale in the servlet response randomly produce a nullpointerexception inside tomcat
• looks like it's related to multithread since it's working well when I wait a bit with a breakpoint on each call to setLocale

Seems to be only in 3.4 branch. Not sure it's all the time but I can surely reproduce it using xwiki-debug-eclipse setup and Denis was able to reproduce it in production too.