changing language for dates in Excel cells

[Marston Conder]

I have a problem with Excel on Mac OS X

As far as I can tell, the language is set as English,
but every time I fill a cell in date format, the date
turns to French,

e.g. 5/04/2004 becomes 5-avr-2004, not 5-Apr-2004.

How can I switch this back to English?

I've checked the language tab under "International"
on System Preferences, and this is definietly set to English

Can someone help me please?

Thanks
Marston