About documents spell checked in English (USA) instead of English (UK), user
The list box under Tools>Options>Language Settings>Writing Aids>Edit is a key to the large window below, allowing you to see modules and sub-modules available for the language selected in the list box and to turn those modules and sub-modules on or off.
The only reason English (USA) always appears here when you enter this dialog box is because it happens to be the first language in the list box, not because this dialog box is setting the language.
Language in an OOo document is set as part of styles in OpenOffice.org. You are probably attempting to spell check in documents where the Default paragraph style and some or all other styles have the language attribute of "English (USA)" rather than "English (UK)".
Press F11 to bring up the Stylist. Select Default from the list, right-click it, select Modify, select the Font tab, and change the language setting for that style to English (UK). This should mostly cascade down to other styles. (You can check for any other styles you are using). If you have been doing direct formatting, you may also have to select your entire document (CTRL-A), select Format>Character>Font and set the language to English (UK) here also...
And to set a language by default for all the future documents, user
...To set things up properly, create a new blank document, set the language to English (UK) under Tools>Options>Language Settings>Languages, and then set the Default paragraph style as outlined above, if it is not already so set. Also check the character styles in the Style list under the A icon in the Stylist, then do File>Templates>Save, giving it suitable name for your normal template, such as 'Normal-Writer' or 'Default-Writer' or some such. Next go to File>Templates>Organize, find your template under My Templates, select it, right-click, and choose Set As Default Template. All new documents will now be created from that template with any style settings you have saved to it, including language settings.
Of course documents not created by yourself may have different language attributes assigned to text or documents you have previously created by have other language attributes assigned to some or all of the text.
from http://www.8daysaweek.co.uk/forums/viewtopic.php?t=758