Cannot change default language to english

When started for the first time, JIRA was speaking Italian (I think because of the Java VM default settings). I am now trying to force the english translation for every user using the global settings, but it seems to have no effect. You can see from the attached screenshot that the default language is reported to be "English" but many translations are still in Italian.

I tried different browser (ie, firefox) and different browser's language settings (italian first, english first, none at all). I also disabled the browser cache.