It won't concern US readers but I had a problem where the language in Outlook defaulted back to US English. This was in-spite of setting the default language in other Office applications. The solution turned out to be setting the language in the separate Office Tools application rather than within the Office Application itself.
- In Microsoft Windows XP, on the Windows Start menu:
- Go to All Programs > Microsoft Office > Microsoft Office Tools
- Click Microsoft Office [version] Language Settings
- In the "Choose the language that defines the default behaviour in Microsoft Office Applications" section select the language you want, OK to apply