Observed on two difference machines, Windows XP and Windows 7, both with the 
same OpenOffice.org version.

Basically, a document created using File-New-Text Document will have its all-
text language set to English (Australia), as configured in Options - Language 
Settings - Languages, and spell checking functionality works.

A documented created in the Windows Explorer (effectively copied from 
C:\Documents and Settings\All Users\Templates\soffice.odt) will have its all-
text language setting be None and this will not be able to changed. Selecting an 
all-text language from Tools-Language-For all text has no effect. Marking 
sections with a language using the other two options in Tools-Language works 
correctly.

With this latter document open and no text entered, my "Default languages for 
documents" Western setting has become "German (Germany)", as well, which is 
thoroughly confusing me, but might be explained by the below styles.xml 
differences. I'm not sure why loading a specific document should change my 
"default language" settings...

The spellchecker fails to check the latter document. I have the en-AU spelling 
dictionary installed but not the German dictionary. I'm not sure how to install 
the German dictionary (I'm guessing I need to rerun the installer) to test if 
that's the problem.

The good document shows "English (Australia)" in the bottom centre status bar. 
The bad document shows "None".

I compared the contents of the two .odt files, and the only relevant differences 
I could see was in styles.xml:

The two documents have difference language settings for their graphic and 
paragraph default styles:
Good:
fo:language="en" fo:country="AU"
Bad:
fo:language="de" fo:country="DE"

I guess that's the source of the above change in default language, I'm not sure 
it's related.

The other difference is in the "Standard" paragraph style. The Bad document 
contains the following element, the good style contains no elements:
<style:text-properties fo:language="zxx" fo:country="none" style:language-
asian="zxx" style:country-asian="none" style:language-complex="zxx" 
style:country-complex="none"/>

That one seems the most likely offender here, as it has a nonexistant language 
code and country...

I did a quick test by extracting the ODT as a zip, removing the offending 
element from the file, and rezipping it, and it now appears to operate 
correctly, allowing me to change the all-text language, although the all-text 
language defaults to German (Germany) according to the bottom-centre status bar.