Go to Help and Type "Change Default Language"
This is part of it but I doubt that the links contained in it will work.
Every version of the 2007 Office release includes proofing tools, such as dictionaries and grammar rules, for more than one language. To see which languages of the proofing tools are included with your version of the 2007 Office release, see The 2007 Microsoft Office suites localized versions. If your version of the 2007 Office release does not include the proofing tools for the languages you want to use, see Determine if you need a Language Pack.
Office 2007 sets itself to the default language of the computer. Is one of your machines set to American english
Hope this helps.