Microsoft Word 2003?
I keep having to change the dictionary setting when spell-checking to UK English from US English. It didn’t matter how many times I did that, the spell check would flip back immediately to US English.
The way to prevent this happening is to make sure that the normal.dot template is itself set to your dictionary of choice. To do this, open a new Microsoft Word document, cut and paste some text into it from somewhere, and then click on Tools/Language/Set Language and select English (UK). Next, whilst English (UK) is highlighted, and the option Detect language automatically is selected, click on the Default button.
You should get a message indicating that the template.dot template will be changed and that all new documents from now on will use the UK dictionary by default.
Microsoft Word 2007?
If you need to set the default dictionary for Word 2007, please see the following article about Word 2007 and setting the default language.