I am having the same problem and have been driven mad by it for years. If
you get any answers, I would be glad to know! If I get any for mine - see
below for first ones - i will pass them on to you as well. Good luck.
Strangely reassuring to hear it isn't just me.

"Australia1" wrote:

I live in Australia and as such am forever having to add words to the Office
dictionary that are spelled differently in Australia (and the UK, New
Zealand, South Africa and various other English speaking countries for that
matter) and having to constantly change the 'default' language setting to
English (Australia) every time I open a new document. Is there no way to
change my default language for all new word documents from English (US)?