Locales in Xfce4

[Peter Humphrey]

Recently I asked about date formats for the Date and Time panel
extension and got useful help - thanks all. But it seems my problem is
wider in scope - it turns out that Xfce doesn't know about locales, yet
Mozilla-Thunderbird expects to inherit the locale from the underlying
desktop environment. How do I fix this? I can't find anything about it
in any of the documentation.

-- 
Rgds
Peter Humphrey