When you login, everything is correctly localized, with one very visible exception : "Browse Projects" in the upper menu bar is still in english.

When you click on "Browse Projects" it gets you to the overview of all projects in jira (which it should do). When you then click on one project ... click ... page load ... tadah! ... the word "Browse Project" is gone and it gets correctly translated as the german word "Projektübersicht". From then on it stays correctly translated during the whole user session. When you logout and login again the problem starts again.

I can reproduce this error on our server and also on jira.atlassian.com