Because with Outlook 2007 you can have the language settings be english but there is a check box that says spell check in French. That is how it is relavant.
It isnt that it spell checks in Frech but it forces the French language with many options. For the first capital letter and all. So if ther box were checked it would force the spell check to be in french not english.
Go under yout Options and check teh spelling tab. See if you have the french box checked if it is 2007.
There you will see the French forced.